A "completely English" Margaret Thatcher stunned a leading British diplomat at a private dinner in Downing Street when she revealed that her great, great grandmother was an O'Sullivan from Ireland.
According to a new RTE documentary, the 1982 dinner-table conversation led the so-called 'Iron Lady' to make a commitment "to do something on Ireland".
The insight features in a new documentary, Thatcher: Ireland And The Iron Lady, which examines her relationship with Taoisigh Jack Lynch, Charles Haughey and Garret FitzGerald.
It also provides a behind-the-scenes look at key events, such as the IRA hunger strikes and the signing of the Anglo-Irish Agreement in 1985.
In the documentary, Sir David Goodall, a leading British diplomat, recounts a conversation he had with Mrs Thatcher at a dinner held to celebrate the British victory in the Falklands War.
"I said in the course of this conversation, 'Relations between Ireland and Britain are complicated by the fact that so many of us in this country are of Irish descent and although they don't like to say so, so many people on the island of Ireland are actually of British descent'.
"Mrs Thatcher listened to that and then she said, ' I am completely English'. So I said, 'Well, I'm not. I mean, one of my grandfathers was born in Ireland and there are hundreds of thousands of people in this country like me'.
"And she said, 'H'mm, now you mention it, my great, great grandmother was O'Sullivan, so perhaps I'm one-16th Irish'.
"At the end of this conversation, she said reflectively, 'H'mm, if we get back again, I think I'd like to do something about Ireland'."
The programme features interviews with former Tanaiste Dick Spring, Michael Lillis, a senior government negotiator on the Anglo-Irish Agreement, and Martin Mansergh, Charles Haughey's special adviser on Northern Ireland.
It also includes contributions from many of Thatcher's former British cabinet ministers and key advisers, including Charles Powell, her former private secretary, Douglas Hurd, former British Foreign Secretary, and her press officer, Sir Bernard Ingham.
It reveals how, despite efforts to improve co-operation between the two states, the British continued to spy on their Irish counterparts.
In the early stages of the Anglo-Irish discussions, key Irish civil servant Michael Lillis recalls his first encounters with his British equivalent.
"We were very conscious that they were both at the heart of the whole British power system, including security and intelligence. And one of them at one stage told us that the communications codes we had at that time were, I think the phrase that was used was 'easy to penetrate' – in other words they were able to read our messages."
Charles Powell says: "She saw the world in terms of conflict and this (Ireland) was a conflict, more than a source of agreement. It was obviously conditioned her attitude to the IRA, but also to the Irish government, who in a sense were perceived as the enemy."
'Thatcher: Ireland And The Iron Lady' will be broadcast on RTE One on Tuesday, July 2, at 9.35pm