Fianna Fáil's parliamentary party have "fully endorsed" the plan for a partnership with the SDLP.
The news emerged following a private meeting which is reported to have taken place on Tuesday 23 January.
It comes as party leaders Colum Eastwood and Micheál Martin are due to make an announcement about the arrangement in Belfast city centre on Thursday.
Talks of a merger have been reported since Fianna Fáil leader Mr Mártin announced his intention to move the party into the North in 2014, although the arrangement will instead be a shared policy platform.
Speaking to reporters on Wednesday, Fianna Fáil's foreign affairs and trade spokesman Niall Collins said, "You are aware of the discussion we had at the parliamentary party last night.
"An announcement will be made tomorrow by our party leader so I don't want to steal his thunder and indeed the thunder of Colum Eastwood at this point in time.
He continued, "The talks, as you know, have been going on for a period of time and we received a briefing on that last night, and the proposal on the table to form a partnership is fully endorsed by the parliamentary party.
"Tomorrow, our party leader travels to Belfast to make the formal announcement."
He would not confirm if Fianna Fáil will field candidates in any Northern Ireland Assembly or Westminster elections.
He said: "We are not going to get in to that at the moment. Those are issues the party leaders will be able to deal with tomorrow."
In October, Mr Mártin's party was caught in controversy in Omagh, when senior party figures unveiled a Fianna Fáil "candidate" for next year's Northern Ireland local council election without the approval of the leadership.
Party colleagues accused Eámon Ó Cuív and Mark Daly, who said they were acting as Sorcha McAnespy's campaign managers, of embarking on a "solo run".
Mr Ó Cuív, who was frontbench spokesman for regional development, rural affairs and the Gaeltacht, was demoted by Mr Mártin.
The move was announced a day after Senator Mr Daly was stripped of his duties as Fianna Fáil deputy leader in the Seanad for his role in the unsanctioned event in Omagh.