He said: "I'm sure the Provos and dissident republicans would shoot me in an instant if they got the chance, so I keep looking in front of me as well as behind me.
"That's just how it is. It's part of the furniture. But I try and not make much of it and just get on with things."
The report last month from the independent body assessing the position of paramilitaries in Northern Ireland said that the IRA and loyalist terrorist groups still existed, but were committed to peaceful means to achieve their political objectives.
The panel was set up by the Secretary of State Theresa Villiers after the murder of Kevin McGuigan in Belfast, which the PSNI said was carried out by members of the IRA.
O'Callaghan (61) said: "I have no doubt in my mind that there's an IRA structure in Belfast for protection from maverick loyalists, from dissidents and from people who have built up grudges over the years - people who have had loved ones murdered, or who were kneecapped.
"And I think there's an IRA structure in south Armagh for financing. To me, all that is perfectly rational, but I don't think there's an IRA structure worth talking about in Fermanagh, for instance. Basically, I don't know what they would be doing.
"I think the guys who were heavily involved in the IRA will always have a major role to play inside the republican movement and I think they bring a sort of discipline and comradeship."
O'Callaghan was jailed for life in 1990 after walking into an English police station to confess to the murders of UDR Greenfinch Eva Martin and Special Branch detective Peter Flanagan in the 1970s. He was freed under a Royal Prerogative in 1996.
But it later emerged that he'd been a Garda informer for nearly a decade and he said it was his tip-off that led to the seizure of a huge IRA arms consignment from America on board the ship Marita Ann in 1984.
He said he also told his handlers that he'd aborted a plan to kill Prince Charles and Princess Diana at a charity event they were attending in 1983.
A number of O'Callaghan's claims have been ridiculed by republicans, who say he's a fantasist, an allegation he dismisses out of hand.
O'Callaghan says everything the republican movement is doing at the minute is geared towards the next elections in the Republic.
"An awful lot of what will follow will follow from the result of that," said O'Callaghan, who doesn't believe that anyone within the republican movement would seriously argue that the IRA had won the war.
"I don't think that Gerry Adams could persuade anyone that the IRA won, so there is always a danger of things slipping back in some way." But O'Callaghan is convinced that a return to a full-scale conflict won't happen.
"I couldn't see it. Surely you would hope that people would have had enough and there wouldn't be the stomach for it - especially after so long at peace," he said.
He is cautiously optimistic about the future.
"I'm like a lot of people. At times I say, yes, I am hopeful, and that it will all go on being mainly boring.
"But, at other times, I think to myself how long Stormont will last in the way it is going. And then I wonder what next year and the anniversary of the 1916 Rising will bring us."
O'Callaghan has written a book about one of the most revered leaders of the Easter Rising who had been a major influence on him as he grew up in Kerry. It's called James Connolly: My Search For The Man, The Myth And His Legacy.
Connolly, a Scots-born revolutionary, socialist, trade union activist and former British soldier, who lived in Belfast for two years, was executed after the Rising.
The manner of his death sparked fury even among people who hadn't been sympathetic with the cause of the rebels in Dublin.
Connolly had been badly wounded in the fighting and couldn't stand up to face the firing squad, so he was tied to a chair in the courtyard of Kilmainham Jail before his executioners opened fire.
Connolly, who was the last of the Rising's leaders to be executed, has a statue in his honour outside the railway station in Dublin named after him.
And in Belfast, Sinn Fein's offices in Andersonstown also bear his name.
A portrait of Connolly was unveiled in Belfast City Hall four years ago and a plaque was erected at his former home on the Falls Road, which is to be turned into a museum.
Connolly's followers believe his time in Belfast helped to shape his republicanism, socialism and trade unionism - something they say southern commentators repeatedly ignore.
He moved to the city from Dublin, where he'd returned after spending seven years in America.
In Belfast, as in Dublin, Connolly was heavily involved with the Irish Transport and General Workers Union and from his office in Corporation Street he fought for and organised strikes on behalf of exploited workers in the city's linen mills, factories, and particularly in the docks.
Connolly, who had come to Ireland as a British soldier but developed a hatred for the Army after seeing the ill-treatment of Irish people and eventually deserted, was also a prime mover in the famous Dublin lockout, which had the support of 20,000 workers between August 1913 and January 1914.
He helped develop socialist parties in Dublin as well as the Irish Citizen Army, and he was so highly regarded as a leader of the 1916 Rising that Michael Collins said of him that he would have "followed him through Hell".
Republicans who regard Connolly as an iconic figure have condemned O'Callaghan over the book. They see their erstwhile colleague as a traitor, but O'Callaghan clearly doesn't care.
He says he was drawn to Connolly in his teens around the time of the 50th anniversary of the Rising. "I saw a series on RTE called Insurrection. It was clear he was one of the few leaders from 1916 who was modern, international and relevant. He was a bridge for a lot of people to a new way of thinking in Ireland away from all the heavy Church stuff.
"Connolly certainly fascinated me. He would have been the biggest influence on me politically as I was growing up. He fired my imagination."
The town where O'Callaghan was raised, Tralee, was a republican Labour stronghold and always returned a Labour TD, and the local trade union was based in Connolly Hall.
The other leaders of the Rising didn't impress O'Callaghan. "Connolly always seemed a bit more pragmatic, with all his trade union work and his socialism, which I was really into," he said.
But, as the years went on, O'Callaghan says he got involved with "other stuff" as he euphemistically calls his time in the IRA, and it wasn't until a literary agent approached with an idea for a book about Connolly that he decided to re-evaluate him.
He says Connolly was a "parcel of contradictions", which centred on why a revolutionary socialist linked up in 1916 with extreme nationalists who didn't really have a coherent political view.
O'Callaghan added: "I am not a historian or an intellectual, but I wanted to come up with a book about a true believer and to discover what a fanatic looks like, and of all the people of 1916 the one I would have admired most was Connolly.
"He was not like the others. He was dedicated to a cause that goes way beyond what people would ever have contemplated."
O'Callaghan now describes Connolly's legacy as "dreadful", adding: "I don't think his writings have been critically examined sufficiently, because he was seen as a martyr and a great Marxist. In a number of biographies about him you get a sense that writers are about to say he was a bad piece of work, but they always shy away from it.
"I think he was a true believer from the word go, one of those people who were prepared to lay down their lives for a holy cause, which in his case was Marxism, which completely dominated his life and he was virtually a professional revolutionary, whether it was in Scotland, Ireland or America.
"He wanted a global workers' republic, a kind of shining city on the hill where everything in the world would be perfect."
But O'Callaghan says that Connolly was a dour, argumentative man who eventually fell out with everyone and every organisation around him.
"No one was ever good enough, dedicated enough or pure enough for him. I could find nothing to suggest that Connolly ever enjoyed himself and he dragged his family all over the place and they were poor. He was not a nice character," he said.
O'Callaghan also argues that Connolly was not a good negotiator. "He thought that compromise was the pits," he added.
He says that Connolly took part in the Easter Rising as a fully committed Marxist with the intention of using the far bigger forces of nationalism to take over the country.
"But he lost completely, because the State that came into being was very much of a Catholic, nationalist ethos with no Marxist influence," he said.
O'Callaghan says he knows his book will not go down well with Connolly's supporters.
"But I didn't go mad out of my way to be controversial - though I hope it will open a debate."
James Connolly: My Search For The Man, The Myth And His Legacy by Sean O'Callaghan is published by Century at £12.99