Bertie Ahern has said if Northern Ireland's politicians do not share power the British and Irish governments would work to implement the Agreement.
The two prime ministers unveiled a blueprint for devolution
The Irish prime minister told Sky news they want to restore devolution.
But, he said, if this fails London and Dublin would work on implementing the Good Friday Agreement.
"Plan B ignores the politicians of Northern Ireland and the deals and co-operation and partnership basis between the two governments," he said.
"We would have to do that because we're the custodians of the Agreement and we're the stewards of the process - but that is not by a long shot our preferred option.
"What the Irish government wants to do is to work with an assembly, to work with an executive to have the north-south ministerial council, working with northern politicians on these issues and working east-west between the two governments."
On Thursday, the British and Irish premiers said the Northern Ireland Assembly would be recalled on 15 May.
'No joint authority'
A deadline to restore devolution by 24 November was unveiled in the "take-it-or-leave-it" plan, outlined by the prime minister and the taoiseach in Armagh.
Tony Blair and Bertie Ahern said the assembly would be recalled on 15 May with parties being given six weeks to elect an executive.
If that fails, the 108 members get a further 12 weeks to try to form a multi-party devolved government.
If that attempt also fails, salaries will stop.
The British and Irish governments would then work on partnership arrangements to implement the Good Friday Agreement.
Northern Ireland Secretary Peter Hain has said unionists should not be concerned about any change to NI's constitutional position if a deal cannot be achieved.
"There's no question of joint authority or anything like that," he said.
The two governments' deadline for the deal will be written into an emergency law due to be brought before Parliament this month.