It's the perfect place for a secret burial. I'm wading through muddy bogland near Hackballscross, Co Louth, in pitch darkness with the map the IRA man gave me. The rain drives down relentlessly from a moonless sky. The wind shakes reeds taller than I am. My torch provides only limited light. I bump into a tangle of bushes.

It's a remote, haunting landscape that sees few visitors. It's here that Gerard Evans' body lies, the IRA man says. Gerard Evans, whom for 30 years the IRA has denied abducting and murdering. Gerard Evans, who was only 24 years old when he left a dance in Castleblaney and was disappeared.

"We held him for three days," the republican says. "He confessed very quickly to being an informer. He wasn't tortured. He pleaded for mercy, he pleaded not to be killed, and then he said his prayers. He was shot once in the back of the head."

The IRA man's directions to the location of the secret grave are precise: "At Hackballscross, take the road to Knockbridge. Pass Kirk's Crossroads (known officially as Maghereagh Cross); continue until you see a narrow lane on the left. Watch the road carefully or you'll miss it. Walk up that lane, past the green field on the right and the stone mounds, and you'll see the bogland. After 150 yards of bogland, stop. Walk 30 yards into the bog – that's where the body is buried."

It was in the dark of night – a night like this, the IRA man recalls – when Gerard Evans was taken to this godforsaken place. He was brought here alive. A dead body would carry too many risks if the car was stopped by gardaí. When they got to their destination, the IRA men ordered him out of the car and marched him across the bogland. What was going through Gerard's head as he was told to walk in front? They gave him a few moments to prepare himself. Then they shot him from behind.

Six miles down the road and across the border in Crossmaglen, south Armagh, Mary Evans, 76, sits at home, desperate for information about her first-born child. "I don't want recrimination or revenge. I don't want anybody arrested or jailed," she says. "All I want is to bring my son home. Everything stopped for our family 30 years ago. We haven't been able to move on."

Mary speaks of her son with pride: "He was a quiet lad. He liked the darts and the snooker and going fishing with his father." He was a good singer too. Mary was never in the pubs herself but she'd hear that Gerard would often take the floor at a session.

Gerard Evans was last seen alive at 11.30pm on 25 March 1979, by the roadside outside Castleblaney, Co Monaghan, trying to hitch a lift home. He'd been at a dance in the Embassy ballroom. He was an unemployed painter and decorator.

Mary has had many crosses to bear. Her husband, also Gerard, never got over his son's disappearance. Fourteen years ago, on his 63rd birthday, Gerard senior walked into Kiltybane Lough and drowned himself. It wasn't the end of Mary's heartache.

"I gave birth to five sons but two have been taken from me," she says. First there was Gerard. Then, her youngest son, Martin, 32, died from a heart attack while visiting Australia two years ago. "People ask me how I manage to smile," Mary says. "But if I didn't smile, I'd cry and I might never stop crying." Another son, Seán, 46, who has Down's Syndrome, keeps her going, she says: "He's with me constantly. He's my great blessing."

'The IRA?are hypocrites'

The IRA has always denied killing Gerard Evans and his neighbour Charlie Armstrong. They lived across the street from each other at Rathview Park. Charlie was abducted in 1981 in an unconnected incident. In 1999, when the Provisionals admitted disappearing nine people, Gerard and Charlie were left off their list.

Gerry Adams later stated: "The IRA has said that it is not responsible for the disappearance of Charlie Armstrong or Gerard Evans. Anyone with information on these two men should come forward to me or to their families."

The IRA man is angry at this denial. "People who stand shoulder-to-shoulder with Gerry Adams when he comes to south Armagh know the IRA killed Gerry Evans. They know where he is buried. Some of them even took part in the execution. I'm not accusing Gerry Adams of anything. I'm not aware of what he personally knows or doesn't know. But I'm saying the South Armagh IRA leadership are hypocrites. The peace process is secure. There's no political reason to keep lying. Just let Mrs Evans give her son a Christian burial before she dies. Put that family out of its misery."

The 'team' that killed Gerard Evans

The IRA man says he knows the full truth of what happened Gerard Evans because he was a member of the unit that killed him. It's the first time anyone involved in an IRA disappearance has ever spoken to a journalist. Contact is initially made through a third party. I'm instructed to go to a pre-arranged spot on the roadside at Lough Ross, a mile west of Crossmaglen, at a certain time of night. I'm told a van will pull up and I'm to get into the back.

Within seconds of my arrival, the vehicle appears. I open the sliding door and climb in. A garden chair is set up for me to sit on. I can see only the back of the driver's head, which is covered in what seems to be a black woolly hat. We travel for 10 minutes. The driver doesn't speak once. It's only later, when we're making the return journey and I see the road signs, that I realise we'd crossed the border into the Republic.

We drive into a lock-up garage. The driver gets out of the van and opens the sliding door to face me. The black 'hat' has now been pulled down – it was a rolled-up balaclava. A middle-aged man in a black jacket, dark jeans and black shoes stands before me. He is well-spoken. The interview lasts 45 minutes.

"I'll try to answer your questions but I won't answer anything that compromises my identity. I'm still an IRA man," he says. "If it got out I was speaking to you, I'd be dead, regardless of the peace process, Stormont, Dublin or London." He admits he's "disillusioned" with the IRA yet remains a member: "If you leave down here, you're treated like an enemy. You're a pariah in the community, you're isolated. That's why I stay."

A dozen people were involved in "the team" that killed Gerard Evans, he says. They had different roles: those who abducted him, those who interrogated him, those who took him to the bogland, shot him, and dug his grave. He refuses to detail his role because that would disclose his identity.

He knew Gerard Evans, he says. He knew the whole family. "Gerry was an ordinary lad but he got in at the wrong end of things. He carried out a robbery and the RUC Special Branch got to him. They told him they'd let him off if he worked for them. He wasn't a big criminal. The robbery was a one-off. He was just looking for a few quid. But now the RUC had compromised him.

"He agreed to be their eyes and ears, to watch specific people, to pick up loose talk in the bars. He worked for Special Branch for about 18 months. He'd extensive contact with his handlers in terms of meetings and phone calls. It wasn't a case that he was barely doing his 'job'. He was well into it. His family had no idea what he was at. They were entirely blameless."

The IRA man refuses to disclose what led the Provisionals to suspect Gerard but says: "He was well on the road to compromising operations, getting volunteers caught and gear captured. He presented a threat to the army in South Armagh."

'Nobody tolerates traitors in a war'

He also refuses to say if Gerard's interrogation was taped but another source tells the Sunday Tribune it was. This source says that, when the IRA abducted Gerard, the telephone number of his handler was in his possession.

The IRA man justifies Gerard's killing: "There was a war on. He could have had volunteers arrested or killed. Nobody tolerates traitors in a war." It was only after the ceasefire and peace process, he says, that he began to believe something should be done about Gerard's body.

"I was stunned when Gerry and Charlie (Armstrong) weren't on the IRA list. I thought the time was right to tell the truth to Gerry's family and give them details of where he's buried. The Evanses weren't an anti-republican family and they didn't become one after Gerry disappeared. They handled the situation in a private, dignified manner. They never went running to the media. They deserve better than they got."

A detailed, hand-drawn map identifying Gerard Evans' burial site has been handed to the Independent Commission for the Location of Victims' Remains. Any DNA evidence from digs, or other information obtained by the commission, can't be used in criminal proceedings. The IRA man didn't meet the commission directly but asked an intermediary to do so.

"I'd been thinking about it for a long time. Then that young lad, Paul Quinn, got killed. I knew the South Armagh leadership sanctioned that but they denied it. Their denial was an absolute lie. I'm sick of lies. A certain morality is OK during a war but things should be done differently in peace."

Paul Quinn, 21, from Cullyhanna, south Armagh, was lured to a farm in Co Monaghan where he was beaten to death with iron bars 15 months ago. The Quinn family blame the IRA but the organisation denies any involvement.

Time for closure

"I don't want the Quinns to have to wait 30 years for the truth like the Evanses. It's time to do the decent thing, to give a family closure," the republican says.

"I don't see why the IRA in other areas can admit disappearing people but South Armagh can't. There's an attitude down here 'we wont admit anything until hell freezes over'. There's this idea, which is ridiculous now, that nobody will break South Armagh, nobody has control over us, you've a duty to keep your mouth shut."

But why didn't the IRA man himself tell the truth years ago? Even before the ceasefire, he could have passed word to the Evanses about Gerard's death or burial spot, either anonymously or through a priest. "I'd more on my mind then. I was concentrating on the war with the Brits," he says.

Wasn't he racked with guilt when he met Mrs Evans or other family members? "From time to time I thought about what had happened but no, it didn't keep me awake at night. I have a conscience but Gerry was a casualty of war. I've no regrets about his execution or secret burial. I'd still defend that 100%."

So why did the IRA disappear and secretly bury Gerard Evans? The bodies of many alleged informers were dumped, hands and feet bound, in bin-liners on south Armagh roads. The IRA publicly admitted these killings and gave its 'reasons'. However traumatic for the victims' families, they were at least spared the mental torture of the relatives of the disappeared. They knew what had happened and were able to have funerals.

But, the IRA man says, the vast majority of informers left by the road in south Armagh weren't local men. They were from other parts of the North, killed by IRA units from those areas. Their bodies were just dumped in south Armagh.

"Nine months before Gerry Evans was disappeared, the IRA killed Paddy McEntee, an informer from Crossmaglen. His body was left in a car and his execution was admitted. The year before, the IRA killed Willie Martin, another local informer, and admitted it. These deaths didn't go down well with some in our community. Privately, there was a bit of a backlash. South Armagh is clannish. Gerry Evans had a large extended family. There are dozens of Evanses around Crossmaglen. It wasn't in our interests to kill Gerry openly just in case we alienated any of them. It was easier to do it secretly. It was an act of self-preservation."

Yet a year after Gerard was killed, the Provisionals shot dead Anthony Shields from Crossmaglen, another alleged informer. The killing was admitted. "It was impossible to deny," the IRA man says. "There were too many witnesses when he was dragged from his car."

It was a mystery in the wider community when Gerard Evans went missing. But the Sunday Tribune has been told that word soon circulated in republican circles that he was killed because he'd agreed to work for the British but hadn't delivered. Rumour had it that the British had shot him and buried him outside Castleblaney, on the shores of Lough Muckno, Co Monaghan.

When Gerard hadn't returned to Crossmaglen by the wee small hours after the dance, Mary was worried. "I kept checking his room. He rarely stayed out. He was a home bird. He shared the room with Seán (his Down's Syndrome brother who was then 17) and Seán would never go to sleep unless Gerry was there. It wasn't like Gerry to be so late because he took great care of Seán."

Still, she told herself, maybe he'd stayed with friends. "A few days passed and he didn't appear. He didn't sign on at the dole either. We went to the police but they knew nothing." Local search parties set out. Charlie Armstrong, who lived across the street and was disappeared himself two years later, was among those who took part.

"There were Chinese whispers about what had happened to Gerry but we never accused anyone or any group," says Noel Evans, Gerard's brother. "We're not interested in these things. We just want people with information about his body to come forward."

For years, Mary believed Gerard might be alive: "I hoped he'd come home after the IRA ceasefire but he didn't. And then when his father and brother died, and there was no sign of him, I knew he was gone. If he was alive, he wouldn't leave us to suffer that pain alone."

Praying for a miracle

Mary still has Gerard's christening robe and shawl. His confirmation picture hangs on the living-room wall. A photograph at a wedding, where he was best man, is lovingly displayed. And up in the attic, in a suitcase, are the clothes from Gerard's wardrobe when he disappeared, she says.

"I've been to Knock and Lourdes many times to ask God that he'll be found. I bring back prayer cards and holy medals and I put them in the pockets of Gerry's clothes, hoping it'll help. For years, I went to Mass every morning. But I took so many knocks, I lost my faith for a while. But you get your strength back. I don't know where from, but you get it back."

In Mary's kitchen, rosary beads hang from a wall. Beside them sits a plaque: 'Guests are always welcome here'. She's a kind, warm woman who offers to make tea and sandwiches countless times. As I'm leaving, she hugs me and gives me a handful of sweets.

This afternoon, Sinn Féin and IRA members will be among the crowd who gather to commemorate Séamus Harvey, 20, a Crossmaglen IRA member shot dead in January 1977. He and Gerard Evans would, at the very least, have known each other to acknowledge on the street. Here was another young man shown no mercy by a different organisation. Lured into a trap by undercover SAS men, his body was riddled with at least 13 bullets.

Among those expected at the monument to Harvey in Coolderry, just yards from Slab Murphy's house, are those who know the truth about Gerard Evans' disappearance but remain silent. And just four miles down the road from the gathering is the bogland where Gerard is said to lie, beneath the reeds shaking in the wind.