Bertie Ahern has formally tendered his resignation as Taoiseach to President Mary McAleese at Áras an Uachtaráin.
Mr Ahern spent the afternoon clearing his desk and bidding final farewells to his staff in Government Buildings, where some senior members will be stepping down with him.
He arrived at the Áras just after 6pm to formally tell President McAleese of his intention to resign.
Mr Ahern has one final engagement tomorrow morning when he leads the Government at the official commemoration of the Easter Rising at Arbour Hill.
Mr Ahern has often spoken of his admiration for the men and women of 1916 and his supporters will find it appropriate that this will be his last official act after 11 years as Taoiseach.
The Dáil will meet tomorrow to elect Brian Cowen as Mr Ahern's successor.
Ahern, Paisley open Boyne centre
Earlier, the Taoiseach and Northern Ireland First Minister Ian Paisley officially opened the Battle of the Boyne site in Co Meath.
Watch the opening ceremony here.
Hundreds of invited guests gathered at the Oldbridge Estate for the event at 11am this morning. They included Orangemen and Apprentice Boys.
Oldbridge is the location of the main battlefield of the Battle of the Boyne.
The 500-acre site, which borders Co Louth, was bought by the State in 1999 to preserve one of the most famous sites in Irish history.
Mr Ahern and Mr Paisley jointly cut a ribbon officially opening the site using swords dating back to the 17th Century.
It was a ceremony to suggest Ireland is at peace, the final act of reconciliation before Mr Ahern stands down as Taoiseach and Tánaiste Brian Cowen succeeds him.
The two leaders will also tour the new facility, which has cost more than €10m to refurbish.
Its refurbishment is expected to almost treble the number of tourists who visit - last year more than 30,000 people visited the site.
Look back at highlights from Bertie Ahern's political career.
Look back at highlights from Ian Paisley's political career
Ahern could run for president: Paisley
Former SDLP leader John Hume has said he never thought he would see the day when Ian Paisley and Bertie Ahern would meet on the site of the Battle of the Boyne.
Mr Hume was among the guests at the opening of the site this morning. He said today's events were evidence of the fact that the past is behind the people of Ireland north and south.
He said it was an important and historic day.
The Nobel Prize winner said Mr Ahern had made peace in Northern Ireland a priority during his time in office. He said his reputation as a peacemaker went before him.
Speaking on RTÉ Radio's Morning Ireland, Mr Paisley said he thinks Mr Ahern will be a candidate for the presidency.
The Northern Ireland First Minister also said the departing Taoiseach could go to Europe.
Mr Ahern had invited the DUP leader to the battle site outside Slane at a meeting in Farmleigh in April.
The Battle of the Boyne was fought between William of Orange and Catholic King James II in 1690. The Protestant Orange Order celebrates William's victory on 12 July.