North reaction: Unionists, all of whom said they had no warning of the men's return, reacted with anger.
The DUP claimed to have been tipped off by a British source shortly before the story broke, while the Ulster Unionists knew nothing until approached by The Irish Times yesterday afternoon.
DUP deputy leader Peter Robinson said: "I have spoken to our government this evening expressing our grave concern about the 'Colombia Three' arriving in the Republic of Ireland."
He said he asked them to convey in the strongest possible terms to the Irish Government that allowing "these fugitives from justice to live freely within the Republic will be an intolerable affront.
"We are continually told that there must be a unified, worldwide response to international terrorism," he said.
"We are looking for their immediate arrest and for the Republic to hand them over to the Colombian authorities. Mr Ahern must facilitate by whatever means necessary their swift extradition. If he does not, unionists in Northern Ireland will believe him to be complicit."
Mr Robinson claimed the men were "intricately involved in the global terrorist network" and he advised Mr Ahern to remember the words of President Bush when he said "those who harbour terrorists are terrorists".
"If Bertie Ahern fails to hand these men over, then in President Bush's and our terms, he is a terrorist collaborator. We demand the clearest statement from our own prime minister that if these fugitives put one foot in Northern Ireland they will be arrested pending extradition proceedings and if the Colombian authorities seek extradition, it will be granted."
Sinn Féin MP for Newry and Armagh Conor Murphy welcomed the men's return: "This evening's news that Jim Monaghan, Niall Connolly and Martin McCauley have returned safely to Ireland will come as a huge relief to their families and to all those who have supported the Bring Them Home Campaign over the past four years.
"Since 2001 the men's lives have been in grave danger and they had their prospects of a fair trial completely undermined due to prejudicial comments from the Colombian military and senior politicians including the then president.
"During the lengthy trial the prosecution case collapsed before the world's media and the trial judge found the men innocent of all serious charges. This verdict was disgracefully overturned by a secret appeals procedure."
Responding to comments from Mr Robinson and Fine Gael leader Enda Kenny, Sinn Féin president Gerry Adams added: "I want to make clear that this issue was not discussed with the Irish Government, nor was it either a deal or a side deal related to recent developments in the peace process."
However, unionists claimed a deal had been arranged between republicans and the Government.
Senior Ulster Unionist Michael McGimpsey said: "These men are still wanted for crimes in Colombia. The Irish Government have a duty to send them back to face due legal process", but he admitted this was unlikely.
He accused the Government of having an arrangement with republicans.
"It is surely no coincidence that the 'Colombia Three' have arrived home after the events of the last fortnight emanating from the IRA statement. The Irish Government are unlikely to send them back, as this move is likely to be part of the latest concession choreography to republicans.
"The 'Colombia Three' are probably the first 'on the runs' to be given a pardon. Once again, despicable political expediency is replacing common sense and due process."
The SDLP's policing spokesman Alex Attwood said: "These men were clearly up to no good. They served time for travelling on false passports. They jumped bail. Above all, people should not forget the huge damage that they did to the peace process.
"Their return will not be celebrated by democratic nationalists across the island."
The Northern Ireland Office swiftly denied the unionist allegations that the return of the three was in any way linked to the peace process, or formed part of any arrangement with any party following the IRA declaration that its campaign was formally over.