The 26-County Taoiseach Brian Cowen crossed the border to Crossmaglen, County Armagh on Thursday night for the formal opening of Fianna Fail’s first offices in the Six Counties.
Mr Cowen is the first serving Taoiseach to visit the town in south Armagh’s republican heartland.
He was one of more than 100 guests who attended a formal dinner at the Cross Square Hotel for the launch.
Fianna Fail officially launched north of the border last year after members voted unanimously to adopt a motion to formally establish branches in every county in Ireland.
The party said it would enter future elections in the Six Counties.
But, for reasons which remain unclear, the party has constantly played down the move, or even denied it outright.
Fianna Fail member and former Sinn Fein Assembly member, Gerry McHugh, who attended the Crossmaglen events, said they were “very significant in political terms”.
The County Fermanagh politician, who now sits as an independent in the assembly, said it was another first for the “historic village”.
“Brian Cowen is the first Taoiseach to come into Crossmaglen, let alone open an office here,” he said.
“It is the continuation of a conversation going on at senior level, and indeed at every level, within Fianna Fail members.
“It is the senior republican party on the island of Ireland and this is a natural enough situation that will develop on from here.”
However, Irish American Democrats blasted Fianna Fail for having failed to to pursue a united Ireland.
Resolutions supporting Irish unity have been passed in 14 cities including San Francisco, Chicago, Philadelphia and Pittsburgh, and in states including California, New York, New Jersey and Massachusetts.
John O’Riordan of the Irish American Democratic Club, which is leading the initiative, said: “If we cannot rely on political leaders in Ireland, then we will develop our own campaign.”
He was particularly critical of Fianna Fáil: “According to its constitution, its goal is ‘to secure in peace and agreement the unity of Ireland and its people’.
“But where are the initiatives? How many years of power do you need to develop a coherent strategy around the goal? Brian Lenihan referring to people shopping in Newry as ‘unpatriotic’ put the lie to ‘the republican party’.”
O’Riordan stressed the campaign was to achieve unity by purely peaceful means. He pointed out that partition hindered economic development both north and south and said he believed US business leaders would be “more willing to invest in Ireland if the country is united”.