Should I believe what MI5 allow me to hear? It's a very simple question for Judge Peter Smithwick.

However, like all things involving the murky world of spooks, simple questions come wrapped in shrouds of justice and deception.

And where lay the facts underneath these shrouds? Do you – can you - see all the facts? Or is it just another shroud covering something else?

How Judge Smithwick answers these questions will have a huge influence on his Tribunal of inquiry.

The role of MI5 has come to prominence again after the Police Service of Northern Ireland returned to the tribunal this week and effectively said 'we've hidden important intelligence material from you’.

It is clear that they could tell the Judge this information now because MI5 said they could.

After years of private investigation the Tribunal began its public hearings and focused on three former gardaí as possible moles who provided the IRA with information leading to the deaths of two senior RUC officers.

Chief Supt Harry Breen and Supt Bob Buchanan died in March 1989 minutes after leaving a meeting in Dundalk Garda Station.

The three former garda sergeants, Owen Corrigan, Leo Colton and Finbarr Hickey, all deny the allegations of collusion.

The PSNI intelligence gathered since the tribunal started seven years ago which said there was indeed a mole but it was none of these three former gardaí.

It also claimed the now retired officer received large amounts of money not only for the information which assisted in the ambush but also for other intelligence.

In fact he may have told the IRA that a farmer from Cooley, Tom Oliver, was passing information to the gardaí leading the IRA to abduct, torture and murder him.

That officer has never been questioned let alone charged.

So the PSNI Special Branch officers got this intelligence.

It was graded as accurate and reliable. The information is passed to MI5 who are in charge of national security and it is for them to decide who they share it with. It is not shared with the Tribunal until this week.

It seems it was not shared with the gardaí either. MI5 have direct contact with their counterparts in the Irish 'secret service', the Crime and Security Branch of An Garda Síochána.

They have information that a garda actually worked with the IRA to murder policemen and ordinary citizens and they don't share it.

As the information is regarded as relating to national security it is not possible to the PSNI to tell the gardaí about it. Instead they keep the information secret knowing there is a Tribunal investigating the issue of collusion.

While it may strictly be outside his remit, it would not be overly surprising if Judge Smithwick said that both MI5 and the PSNI had, in effect, colluded with the IRA by not passing on this information at the earliest possible date – especially to the gardai so they could investigate the allegations.

Det Chief Superintendent Roy McComb came to the Smithwick Tribunal this week to put a very brief summary of the intelligence before the Judge. It was clear he was under strict instructions not to stray beyond the summary.

However, he had to concede the obvious – the name of this garda would be known to the Special Branch handlers who obtained the intelligence and so could be established.

He said the five pieces of intelligence had only come to light after a review following his previous appearance at the Tribunal.

“Decisions were made that the documentation would not be shared. I am afraid I can't assist you at this point as to who made those decisions or when those decisions were made,” he told Michael Durack, counsel for the Garda Commissioner.

Up to his arrival in the witness box, the PSNI had only provided one intelligence form.

It was from 1985 and it named Det Sgt Owen Corrigan as a garda who was passing information to the IRA.

While the tribunal laboured under the premise that this was the only piece of police intelligence relating to the issue of collusion, some senior PSNI officers and MI5 officials knew differently.

It is important to note that the Special Branch handlers who received that allegation about Mr Corrigan appeared before the tribunal and were be questioned about it.

It seems unlikely the handlers who garnered this new information will do the same.

Jim O'Callaghan, counsel for Owen Corrigan, pointed out the unfairness of what had been done to his client.

He had been “hung out to dry” by the PSNI. They had this information which in his view cleared his client but the security services kept it secret for years.

But the question of whether this could be misinformation also has to be considered.

On his last visit to the Tribunal Det. Chief Supt. McComb acknowledged that there were concerns among republicans about what the Tribunal was at and what it might uncover.

It is fair to say that if this is the case, it is not only republicans who would be concerned... there are others who would be keen not to have too many things uncovered about the Troubles in 'Bandit Country'.

As well as the IRA and other republican groups, this would obviously include RUC Special Branch and British military intelligence.

Which brings up a point raised by Neil Rafferty, counsel for the former British agent, Peter Keeley.

Mr Keeley working in the IRA for the British security services for several years.

The PSNI officer agreed that it would be possible for misinformation to be put out by the IRA, picked up by the Special Branch handlers and fed into the intelligence network.

It is also possible that misinformation could be spread by the security services themselves to suit whatever purpose they want.

Sifting information from misinformation is one of the key roles for MI5 and now also for Judge Smithwick. At least they have that much in common.