The Smithwick Tribunal is expected to seek a third extension of time after it emerged that one of its key witnesses is awaiting a heart bypass operation.

Consultant cardiologist Niamh Murphy told the Tribunal today that Owen Corrigan - one of the three former gardaí being investigated by the Tribunal to see if they colluded with the IRA - is unfit to give evidence at present.

She said he will not be in a position to do so for a further five to six months.

The Tribunal was set up in 2005 to investigate claims that gardaí were involved in passing information to the IRA which led to the murders of RUC Chief Supt Harry Breen and Supt Bob Buchanan in March 1989.

They died in an IRA ambush just minutes after leaving a meeting in Dundalk Garda Station.

The Tribunal began gathering information in private in 2006 and has been hearing evidence in public since May 2011.

It had been due to complete its work by November of that year but was given an extension of time to early 2012.

Then in May the Tribunal sought another extension to November, which was approved by Minister for Justice Alan Shatter following a brief spat with Judge Smithwick.

Now the Tribunal is expected to write to the clerk of the Dáil to seek another extension, possibly to late spring or early summer of next year.

Previously, the judge had said it would take three months to write his report with a further month for editing and printing.

The Tribunal is focusing on the role of three former sergeants, Owen Corrigan, Leo Colton and Finbarr Hickey. All three deny the allegation of collusion.

Today, Ms Murphy told Judge Smithwick that Mr Corrigan was ill at the moment and was recently released from hospital.

She said he was currently at home, self-medicating and was on a course of heavy antibiotics. After that he would be undergoing a heart bypass operation.

Ms Murphy said her patient would not be in a position to return to give evidence until next year.

The witness said Mr Corrigan would be at risk of a heart attack if he came back to the Tribunal before having a bypass.

senior counsel for the Tribunal Justin Dillon pointed out that Mr Corrigan was being led through his evidence by his own legal team and that would not be overly stressful.

He asked would it be possible for the witness to give evidence via video link from his own home.

However, Ms Murphy said the stress was not related to where he was giving evidence but to the fact he was giving evidence before a Tribunal.

She agreed she would inform the Tribunal if his condition improved earlier than the predicted five months recovery time.

Counsel for Mr Corrigan Jim O'Callaghan pointed out that most of the legal teams at the Tribunal had already questioned his client.