Ex-IRA chief of staff Kevin McKenna, who was in his 70s, died on Tuesday in Monaghan.

Former IRA man Sean O’Callaghan said McKenna’s “hate” convinced him to turn his back on terrorism, later becoming reportedly the most senior Garda agent ever inside the IRA.

Born on the family farm near Aughnacloy in 1945, McKenna was arrested in 1972 while IRA commander for Tyrone. When released three years later he moved to Monaghan to reassert control, living there ever since and helping to run a family pub.

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A long-time Sinn Fein supporter, he was reportedly often a bitter rival of Martin McGuinness.

Mr O’Callaghan’s friend, historian Ruth Dudley Edwards, said McKenna’s attitude had a huge impact on Sean.

She said: “Kevin McKenna was someone Sean trusted and looked up to and yet he was also the person that suddenly laid bare to him the horrid squalid sectarianism at the root of what the IRA were doing. It was the final straw for Sean really, it never left him.”

In his book, The Informer, Mr O’Callaghan expressed shock at the “deep and ugly hatred” of the IRA campaign he joined in Co Tyrone in the 1970s.

Where O’Callaghan would prefer to attack the British Army, he said the local IRA men “would rather shoot a Protestant neighbour who was in the UDR or police reserve”.

He spoke of being in a safe house with McKenna in 1975 when the television news reported a policewoman had been killed by a bomb.

Mr O’Callaghan wrote: “McKenna turned his head slightly in my direction and said: ‘Maybe she was pregnant and we got two for the price of one’.”

He added: “How the f*** could anyone hate so blindly? And this man was second in command of the Provisional IRA.”

Already having doubts about the IRA, he retreated to a bedroom where he “burst into tears” as he realised that there was now “nowhere to go ... McKenna had answered all my questions”.

A retired police officer who served in Tyrone and Fermanagh said that as chief of staff from 1983 to 1997, McKenna would have led the murder of “countless” civilians in bomb attacks such as Enniskillen, the Shankill Road, Teebane and Canary Wharf.

He also would have led, he said, the strategy of murdering civilians working for the security forces, using some of them, such as Peter North and Patsy Gillespie, as “human bombs”.

Sinn Fein MP Michelle Gildernew said McKenna had been “a good friend and mentor and good to me and someone I would have looked up to and went to for advice and guidance”. He had been “totally wedded to the peace process and trying to bring about Irish freedom and equality” she told the Irish News.