A meeting between Prince Charles and Gerry Adams today is only going ahead because the Queen insisted, the Belfast Telegraph has reported.
The main political parties in the Republic had strongly objected to the behind-closed-doors meeting on the campus of the National University of Ireland, Galway.
But they were forced to back down at the 11th hour after Buckingham Palace officials insisted that it went ahead.
One senior London source familiar with the negotiations said: "The resistance of the southern Irish Government was eventually broken by the enthusiastic support of the Queen and Prince Charles.
"The other party leaders will get a private five-minute meeting to provide political cover."
The historic meeting, which also involves Deputy First Minister Martin McGuinness, will be held away from cameras, making a handshake more likely between the republican leader and the royal whose favourite uncle Lord Louis Mountbatten was blown up by the IRA.
With Dail elections to take place next year, and TDs concerned about Sinn Fein 'hijacking' Easter Rising centenary events, the parties in Dublin had fiercely opposed Mr Adams being portrayed as a peacemaker in the south.
However, sources said that after her groundbreaking handshake with former IRA commander Mr McGuinness at Belfast's Lyric Theatre in 2011, the Queen wanted to make another political gesture. A Buckingham Palace source said: "The past is the past. It is time to move forward."
And Sinn Fein chairman Declan Kearney said: "This was agreed to promote the process of resolving past injustices and promoting reconciliation and healing."
The meeting with Mr Adams comes ahead of a visit to the Sligo site of his great-uncle's murder and is widely seen as the next phase in ever deepening relations, friendships and bonds between the UK and Ireland.
Charles, who will be accompanied by wife Camilla on his first official visit to Ireland in 13 years, will make an emotional trip to Mullaghmore tomorrow where 79-year-old Mountbatten was killed in a 1979 IRA bomb attack as he set off in a boat with others to check lobster pots.
The couple will arrive in Northern Ireland on Thursday.