It all began on the Friday before Christmas, when those not out at office parties watched Áine Tyrell on UTV's Insight programme detail how her father, Liam Adams, had allegedly raped her from the age of four. It was a horrific story, but nobody could have guessed the huge political cover-up that lay behind it.

Gerry Adams appeared on the programme saying he had believed Áine from the moment in 1987 that she told him what his brother had done. He had always supported her, he said. Tyrell's interpretation differed. She insisted he had failed her.

But it wasn't Tyrell's words that meant this story was far from over. It was what Gerry Adams did and didn't tell reporter Chris Moore that would lead to claims of a huge cover-up by the Sinn Féin president.

Adams told Moore that, after hearing Tyrell's allegations, he'd been estranged from his brother for 15 years. Two days later, the Sunday Tribune proved that was far from true. We published photographs of the Sinn Féin president at Liam's wedding to his second wife almost 10 years after he had been told Liam was a paedophile.

To attend a family funeral, where Liam might also have been present, would have been understandable. But to attend Liam's wedding – to stand smiling and relaxed with him at the reception wearing a green Saoirse ribbon for IRA prisoners – was another matter. Gerry Adams obviously had a very odd understanding of the word 'estrangement'.

But that wasn't all we revealed. Gerry Adams never told UTV's Chris Moore – on tape or in any pre-recording conversations – that his brother Liam had been in Sinn Féin. His account to Moore was neither honest nor transparent.

Two days later, the Sunday Tribune revealed that Liam Adams had been a high-profile Sinn Féin member in Dundalk in the 1990s. So senior was Liam Adams in Sinn Féin that he had sought the nomination to be the party's Co Louth candidate in the 1997 Dáil election, but had failed. The nomination was secured by local veteran republican Owenie Hanratty at a selection convention in the Imperial Hotel in October 1996.

And we exposed something else. UTV had reported that Liam Adams had worked for youth projects in Belfast. The Sunday Tribune revealed he'd also worked for a youth project in the Muirhevnamor estate in Dundalk.

Hours after our revelations, RTE broadcast an interview Gerry Adams gave to RTÉ's Tommie Gorman. Gerry Adams admitted Liam had been in Sinn Féin. He also told Gorman how his own father, Gerry senior, had been a paedophile and had abused family members. This revelation successfully diverted attention away from questions the Sinn Féin president would have had to answer about his own behaviour.

The media focused on Gerry Adams, the victim. It was strange that Adams had never once mentioned – on tape or in pre-recording meetings – his father's paedophilia to Chris Moore in the UTV programme when it would have been entirely relevant. To reveal it in a broadcast later smacked of news management and damage limitation.

The day after the Gorman interview, Adams told RTÉ that, on hearing Liam was in Sinn Féin, he "moved immediately" to stop his Dáil nomination and "to get him dumped out of Sinn Féin... I moved very, very quickly".

The following Sunday we printed material that proved that he was lying. We published photographs of Gerry Adams canvassing in June in the 1997 Dáil election campaign with the brother he believed was a paedophile and from whom he was allegedly estranged. This canvass occurred eight months after Gerry Adams said Liam had been "dumped" from the party.

The Adams brothers were seen laughing on the canvass in a shopping centre and on Dundalk streets. Liam was proudly sporting a Sinn Féin badge. Liam's Sinn Féin colleagues from Dundalk gave us details of his extensive involvement in the party and how he "breathed new life into the Muirhevnamor cumann", setting up many community schemes and always talking about his brother.

We also revealed that it wasn't Gerry Adams who stopped Liam becoming the party's Dáil candidate. Republicans present at the selection convention said Liam Adams had entered the convention, which was chaired by Caoimhghín Ó Caoláin, as a candidate. But when he saw that the crowd was strongly in favour of his rival, Hanratty, he stood up and announced he was withdrawing from the race. Minutes were taken of the meeting. Again, Gerry Adams had lied.

Sinn Féin Louth TD Arthur Morgan rushed to his leader's defence. Liam Adams had been a member of Sinn Féin, he admitted, but he'd played a minor, short-lived role. He "was never a party officer".

The following week's Sunday Tribune proved that was completely untrue. We revealed that Liam Adams had been Sinn Féin's most senior official in Co Louth. He was chairman of the Louth comhairle ceantair, liaising directly with the leadership. Morgan would later claim this had "slipped his memory".

We published a photograph from the Argus newspaper of Liam Adams, as Louth comhairle ceantair chairman, standing beside Martin McGuinness at the official opening of Sinn Féin's new office in Dundalk in June 1996.

And we had another revelation. Thirteen months after Liam Adams was supposedly "dumped" by his brother from Sinn Féin, he chaired the Edentubber Martyrs' commemoration in Co Louth, attended by 1,000 republicans. He introduced Sinn Féin's then national chairman, Mitchel McLaughlin, as the main speaker at the event, which is one of the most important in the republican calendar.

But even that was not the full extent of Sinn Féin's cover-up. Not only had Liam Adams never been expelled from Dundalk Sinn Féin, but when he left the town he went on to be highly active in Belfast Sinn Féin.

A republican gave us a detailed statement of how Liam Adams was "an active and founder member of the Lower Andersonstown Sinn Féin cumann by the name of 'Cumann Mheon na Fuiseoige'". That cumann met weekly in the Felons' Club, two streets away from Gerry Adams' home.

A fellow cumann member was the wife of a senior Sinn Féin official who is extremely close to Gerry Adams, the republican told us. It was inconceivable the Sinn Féin president didn't know his brother was a leading figure in the party in the heart of his own constituency. The republican revealed Liam Adams had canvassed and fundraised for Sinn Féin in Belfast.

On Thursday 14 January, the Sunday Tribune sent Sinn Féin a list of 25 questions about Liam Adams. For the first time in the media, the possibility was raised of Liam's involvement with Sinn Féin in Belfast.

In another damage limitation exercise, Sinn Féin the next day admitted that Liam had been chairman of a west Belfast cumann but claimed Gerry Adams didn't know this at the time and had found out only 24 hours earlier.

In reply to our questions, Gerry Adams admitted Liam had been a treasurer for Sinn Féin in west Belfast and a "joint signatory" on the Andersonstown cumann chequebook. The party also said Liam had been a party member in Donegal in the "mid-1980s", and while he hadn't been a member of Dublin Sinn Féin, he had attended "republican events" in the city.

We also revealed that Gerry Adams had contravened his own party's constitution. We were given a copy of the constitution at the time when Liam was active in the party and Gerry Adams believed him to be a paedophile. The constitution states: "Where allegations of sexual harassment and sexual assault are made, they should be referred directly to an ard chomhairle."

Gerry Adams had previously insisted he told no one else Liam was a suspected paedophile. The most powerful man in Sinn Féin had treated his party's rules with contempt.

During our investigation, Sinn Féin has accused the Sunday Tribune of "gutter and tabloid journalism". Mary Lou McDonald said we should hang our heads in shame.

But it's only through our investigation that at least some of the facts about Liam Adams' political career have been revealed and that morsels of truth have been dragged, kicking and screaming, from a party which is in government in the North and which seeks the same in the Republic.