Fianna Fáil's decision to look at organising in Northern Ireland is unwanted and destabilising, the Ulster Unionists have said.
Reacting to the announcement that Minister for Foreign Affairs Dermot Ahern is to chair a committee examining closer Northern links, UUP leader Sir Reg Empey said any move by Fianna Fáil north of the Border would damage attempts to achieve political normality.
He accused Taoiseach Bertie Ahern of throwing a hand grenade into the middle of Northern politics.
"The last thing we need is another abstentionist party organising here," he said. "Bertie Ahern at a stroke has transformed himself from a potential partner in the process of normalisation, to a rival."
Looking to the possibility of the election to Stormont of Fianna Fáil candidates, Sir Reg added: "The prospect of Fianna Fáil ministers being in both the Northern Ireland Executive and the Dublin government could put unbearable strain on the political process before it has had a chance to settle down."
The plan was "a most unwise and unhelpful decision from Bertie Ahern" which would "fuel demands from unionists to seek further links with mainland parties as a counterbalance".
The raising of the issue "at a time when all our energies are directed at achieving stability", would undermine the progress achieved to date and fostered by the two governments, he said.
SDLP leader Mark Durkan welcomed Fianna Fáil's announcement as evidence of new post-Belfast Agreement politics, telling The Irish Times he was aware of it in advance.
"An Taoiseach indicated that Fianna Fáil is approaching the questions of island-wide politics," he said. "For our part, the SDLP has also been considering this in the context of a review of party development. Indeed, members of the review team recently met Dermot Ahern TD as well as members of other parties in the South."
"We welcome today's announcement and look forward to continuing our engagement with Fianna Fáil through Dermot Ahern's new working group, where we will set out our vision of the future development of national politics."
He said that the SDLP "as a true republican party" believed that the social and economic interests of the people of both parts of Ireland would be best served "by ever-deepening co-operation between North and South".
"We anticipate a healthy and forward-looking debate as a means to forging a new political path for the whole country."
Sinn Féin Assembly leader John O'Dowd welcomed Fianna Fáil's decision. "It is encouraging that the Fianna Fáil leadership is finally incorporating an all-island approach into its political priorities and strategies," he said.