It was nerve-wracking. I've sat in court on countless occasions waiting for judgments. But always as a journalist reporting on other people's cases. This time it was my fate being decided. The Sunday Tribune and myself against the PSNI. Most people thought we hadn't a chance. It was just three months after the Real IRA murdered two British soldiers at Massereene. Detectives wanted my computers, phones and notes relating to stories on the dissidents.

The police were said to be absolutely confident they'd win. I'd already decided that, regardless of any court order, I couldn't cooperate even if non-compliance meant up to five years in jail.

It took judge Tom Burgess 30 minutes to read his detailed judgment. At last, came the all-important words: "I have concluded that in this case, taking into account all of the factors... that the application for the production order should be refused."

It was a landmark decision. Burgess recognised that not only would handing over the material place my life in danger from the Real IRA, but that the very concept of journalistic confidentiality regarding sources was protected in law.

Media colleagues, who had crowded the public gallery, were delighted. It was far better than anybody had hoped. "I can't believe it's that good, Joe!" I said hugging my solicitor. The police looked shell-shocked, like they'd never even countenanced they could lose.

While relieved, I remain perplexed as to why I was hauled into court in the first place for doing nothing other than my job.

It began with a simple telephone conversation, the evening after Massereene. I was in a Belfast shopping centre, about to go into Sainsburys, when a Real IRA spokesman made contact claiming responsibility for the attack. He used recognised codewords.

I abandoned the shopping, ran home, and phoned other media to put the information into the public domain. It was nothing different than what hundreds of other journalists have done in the North over the years. I've personally taken many previous paramilitary claims.

I interviewed the UDA a week after they murdered six Catholics and threatened to murder more. I've received Provisional IRA statements in cafés and ice cream parlours. I conducted a lengthy interview with a Provisional IRA figure, who is now a Sinn Féin Assembly member, in a house in Andersonstown.

Not once did I hear from detectives afterwards. So why now? Colleagues were equally stunned. The Press Association's Ireland editor, Deric Henderson, told me he'd taken the Provisional IRA claim of responsibility for the 1984 Brighton bomb in which five people were killed, and Margaret Thatcher and her cabinet narrowly escaped death.

"I met a republican who handed me a typed statement admitting the attack. I'll never forget it because the paper was pink," Henderson recalled. Police never even contacted him, let alone asked for the original statement which may well have contained fingerprints.

It was a non-descript middle-aged man in a suit who knocked at the door of my Belfast home and handed a letter to my partner, who hadn't a clue who the man was, nor what the correspondence contained. The letter warned that if I didn't hand over my journalistic material to police, they'd ask for an order under the 2000 Terrorism Act.

It quickly became Kafkaesque. At the first hearing, the public, press, myself and my lawyers were cleared from the court so the PSNI could present their arguments in camera. I was appalled. How could we mount a defence when we didn't even know what police were saying? We were fighting a case from a massively disadvantaged position.

And so we decided to meet the challenge head-on. This wasn't just about the PSNI against one journalist and newspaper – there were huge ramifications for the entire media. For the first time in a source-protection case, a range of eminent journalists would be called to give evidence and defend our profession's principles and practices.

If the PSNI case was shrouded in secrecy, we would respond with transparency. I would go into the witness box and be open to intensive cross-examination by chief constable Hugh Orde's barrister. The passion and commitment of our legal team – Joe Rice, Conor Houston, Peter Girvan and Arthur Harvey QC – was magnificent. A strong case was built under trying circumstances in a short space of time.

The National Union of Journalists launched a petition. I spent 12 hours a day or more – locked in the office and rarely seeing my 14-month-old daughter – phoning and emailing people to garner support. In just over a fortnight we gathered more than 5,000 signatures. They were impressive – actor Stephen Rea, Booker-prize winner Roddy Doyle, singer Christy Moore, award-winning journalist John Pilger, and many more.

The petition comprehensively crossed the political divide, a rarity for Northern Ireland. Ex-IRA prisoners and ex-IRA informers; civil liberties groups and former senior detectives; the Bloody Sunday families and the father of murdered loyalist leader Billy Wright – they all signed.

The support of former PSNI assistant chief constable Alan McQuillan, who had served side-by-side with Hugh Orde, was invaluable. Willie Frazer of IRA victims' group Fair signed too. "It's the only time Willie's name will ever appear beside Christy's," somebody joked. Even Willie laughed. It was the diversity of the petition which made it powerful: people who shared nothing in common other than cherishing freedom of the press.

During the main court hearing when we presented our arguments, the PSNI's case was utterly unconvincing. Under cross-examination, Orde's barrister Tony McGleenan told me that if handing over material to the PSNI placed my life in danger, I could apply for extra police patrols in my area and security measures for my home.

It was a preposterous suggestion. I told McGleenan if police couldn't protect soldiers outside a military base in unionist Antrim, I didn't have much faith they could protect me living in a mainly Catholic part of Belfast. But I didn't want to live my life behind a phalanx of cameras, bullet-proof glass and reinforced doors anyway.

Nor was entering a witness protection scheme in England tempting. I'd no wish to leave my home, uproot my partner and baby, take on new names and identities, and lose other family and friends forever. I've interviewed enough informers to know what a hellish existence theirs is and how quickly they are abandoned by the state they've loyally served once they outlive their usefulness.

The witnesses for the Sunday Tribune – Panorama's John Ware, Alex Thomson of Channel 4, the Sunday Times' Liam Clarke and former Mirror editor Professor Roy Greenslade – gave superb evidence.

Just before legal proceedings began, the PSNI made me an offer to speak to them privately. I declined. That decision was totally reinforced during Ware's cross-examination.

Barrister Tony McGleenan asked Ware if he knew that one journalist – telephoned several days after the Omagh bomb with a claim of responsibility and apology from the Real IRA – had made a statement to police.

The court heard that the journalist had described the accent of the caller – who sounded like he was from Dundalk – and had given a rough estimate of his age. The journalist had also given police a list of dissident republican contacts whose voice he believed it wasn't.

Ware, the Omagh expert, was stunned by this revelation, as was every journalist in the court. "Is he still working as a journalist?" Ware asked the barrister increduously. In the corridors outside, journalists speculated as to the reporter's identity.

In court, Ware said he could think of no other recent Northern Ireland chief constable who would have gone down the same path as Orde in pursuing journalists. "There is meant to be progressive policing in Northern Ireland. If this is progressive policing, I'd hate to see what regressive policing would be like!" he declared.

The PSNI's inconsistency regarding contact with paramilitaries is mind-boggling. Sinn Féin regional development minister Conor Murphy has said he's met the Provisional IRA and been assured they weren't involved in murdering south Armagh man Paul Quinn. The Provisional IRA remains an illegal organisation. Has the PSNI even attempted to quiz the government minister about his meeting or his sources?

With the Sunday Tribune, the PSNI were perhaps attempting to test the Terrorism Act, to see how far they could push it with journalists (while turning a blind eye to government ministers). If so, their experiment backfired.

Burgess ruled "that the concept of confidentiality for journalists protecting their sources is recognised in law and specifically under the 2000 Terrorism Act and Article 10 of the [European] Convention."

It's the first time since the 2000 Terrorism Act that protection of sources has been enshrined in a judgment. In the only previous case, against Shiv Malik over Islamic sources, police won.

Last week's judgment in Belfast hopefully means the PSNI will think twice before pursuing another journalist. Although legal sources fear that police now might go down the route of increased covert surveillance under RIPA (Regulation of Investigatory Powers Act) – bugging journalists' homes and phones, and filming them meeting sources.

That remains to be seen. For the moment, we should savour a victory secured against incredible odds. Had this case been lost, no journalist would, in practice, ever again have been able to take a paramilitary claim of responsibility.

Vital information would be denied to the public. They would have to guess at the identity of those responsible for an attack, or rely solely on official police statements. As Willie Frazer said: "Much as I loathe the Real IRA, I want to know what they're saying, not what Hugh Orde wants me to hear."

Books about paramilitaries have contributed, nationally and internationally, to understanding the Northern conflict. But had the PSNI won last Thursday, works like those by Eamonn Mallie, Ed Moloney, Martin Dillon and Toby Harnden effectively couldn't be written in future.

Last year, a BBC Panorama team were arrested with dissident republicans in Donegal after interviewing the Real IRA. The journalists were soon released. The dissidents were charged and held in Portlaoise prison, although the case against them eventually collapsed. A senior police officer later complained: "Those f*****s in Panorama should have been locked up with the other scumbags in Portlaoise."

To interview paramilitaries – to put their views and motivation into the public domain – is perfectly legitimate. Explaining their activities is not exonerating them. The attitude of some PSNI figures show they don't respect this.

In Belfast Laganside courthouse last Thursday, there was no evidence of liberal policing. Instead, enlightenment came in a courageous decision by one judge which should have far-reaching implications for press freedom in Ireland, Britain and across Europe. And for that, we must all rejoice.