Perhaps, as he says, beliefs are “moulded into the DNA”, or maybe a lifetime in politics sustaining only mild abrasions has taught him never to be caught off guard, but Bertie Ahern seems to know exactly who he is.

That, he explains at the beginning of the family tree rustling series Who Do You Think You Are? (RTÉ One, Sunday, 9.30pm) is an Irish Republican – a staunch believer in a 32-county united Ireland, a dream imbibed from his fiercely nationalist parents. “As long as none of the relations were too close to the royal family, I’ll be happy enough,” he guffaws.

What he does dredge up, for most of the programme, are corroborating details of his own accounts. His father, Con Ahern, known as Sunny, fought for the IRA during the Irish Civil War but seldom spoke about his imprisonment, first in Cork Women’s Prison, and later in Tintown internment camp, on The Curragh.

A quirk of history comes like a sniper’s bullet when he learns that Tim’s son, Tim jnr, was a Free State soldier killed in an IRA ambush

“He knew to hide the gun,” one historian tells Ahern of his father’s arrest and narrow escape from execution: “If he had any. Not saying he did.”

“He did,” Bertie nods, the snitch.

Con took part in a hunger strike at the camp, after the war had ended and releases were stalled, which affected him “all his life”. “It was a tough life,” reasons his son. “No longer they ended up with the tough beliefs that they did.”

That, however, may also be a genetic trait. Con, we learn, departed the family farm and disavowed his inheritance, never returning home after the death of his own father, which, we learn, was by suicide.

This summons memories of Ahern’s infamous disbelief, in 2007, that economic nay-sayers “don’t commit suicide”. Today he wraps that detail into a tangled apologia: “Some of the people who wouldn’t have loved me said, if you understood it in your family, you wouldn’t have said that. Now, I didn’t bother saying that I understood it very well.”

Okay. But does that personal connection with the worst expression of despair make his blasé statement better – or considerably worse?

If his father’s side of the family holds no secrets, his mother’s seems still more transparent. “She hated the Brits and nearly everything about them,” says Ahern, not unhappily. A safe house for the IRA, her home was raided by the Black and Tans and her father’s life barely spared.

But the discovery of another relative, his great granduncle Timothy Burke, master of the Dunmanway workhouse, proves a deeper vein historical surprise.

At first, Ahern is encouraged to find another politician in the family, and a nationalist to boot. “I would have told him how to get the votes on the first count,” says Ahern, the best, the most skilful, the most devious and the most cunning of time-travelling political advisers.

But a quirk of history comes like a sniper’s bullet when he learns that Tim’s son, Tim jnr, was a Free State soldier killed in an IRA ambush – the side, we are reminded, for which his father fought.

This is a stark reminder of the tragic, bitter divisions of the Civil War, and the legacy to which our leading political parties still cleave. Still, Ahern folds it instinctively into partisan rhetoric: “It confirmed what I’d always thought: that I’d have to go a long way back before I found anyone that was on the Free State’s side.” (That Eoin O’Duffy, founder of the Blueshirts, writes a letter of tribute for the slain Tim, hardly endears him further.)

Never celebrated for his powers of articulacy, Ahern’s equivocal responses here reveal remarkably little about him: the whole experience he concludes is “sad in one way and hugely enlightening in another”.

But to find his family branches so split, with that painful division presumably moulded into his own DNA too, invites some deeper reflection about conflict and reconciliation.

Ignoring it, however, is the persistent fault of history, and of politics; to learn the lesson but miss the meaning.