Northern Ireland's First Minister Ian Paisley and Irish Republic Taoiseach Bertie Ahern have visited the site of the 1690 Battle of the Boyne.
The two men viewed an exhibition on the battle at Oldbridge House, part of a multi-million pound restoration project by the Irish government.
Work on the project in County Meath is expected to be completed next year.
The Battle of the Boyne was fought between William of Orange and Catholic King James II in 1690.
Mr Ahern said: "This is of course a special place for the Protestant/unionist and loyalist people of this island and the government was deeply conscious of this when we acquired the site back in 2000.
"Since then we have launched a development programme which will see the completion of new visitor facilities next year, which will secure over 500 acres of beautiful countryside for future generations to enjoy."
However, Mr Ahern added that the importance of the Boyne site went far beyond "the significance for just one tradition alone".
"Our history is complex and so was the battle fought on this ground. Catholics and Protestants fought on both sides."
Mr Paisley said he appreciated Mr Ahern's welcome and "what has been done here and you will have an invasion from Ulster on many occasions".
And went on to joke: "I trust you will not allow any of these weapons to be commissioned.
"You'll have to have the person in charge of decommissioning on a full-time job to see that all is well."
Mr Paisley presented the Taoiseach with a musket in return for Mr Ahern's gift at the St Andrews talks of a walnut bowl made from a tree from the site.
A tree was also planted in the grounds of Oldbridge House to mark the occasion.
Mr Ahern and Mr Paisley also attended a reception for guests from north and south of the Irish border.
Re-enactments are staged at the site of the battle
The guests included politicians, representatives of the Orange Order and other loyal orders, local authorities and others associated with the project to develop the site.
The Tanaiste - Irish deputy prime minister Michael McDowell - and Minister for Foreign Affairs Dermot Ahern were also in attendance, along with newly appointed ministers from the Northern Ireland executive.
Mr Ahern and Mr Paisley agreed to the visit at the start of last month after talks in Dublin between the two men.
Mr Paisley said it would show "how far we have come when we can celebrate and learn from the past".
The Protestant Orange Order celebrate William's victory every year on 12 July.