Train drivers are expected to serve notice today of a series of one-day strikes that could disrupt services as early as next Monday.
About 100 former members of the Irish Locomotive Drivers' Association (ILDA) have been balloting for the past week on the threatened action, due to begin with a one-day stoppage next Monday.
The drivers are now members of the 3/57 branch of the ATGWU.
Senior officials of the union may decline to sanction the action, but the branch secretary, Mr Brendan Ogle, says the strikes would proceed in any case.
"The only question is whether they would be official or unofficial, and the only thing that turns on that is the question of strike pay."
The branch, he said, would have "quite a number of things to consider" after the ballot was counted today.
Drivers voted on a motion which calls for a series of one-day work stoppages to begin on December 15th and continue on dates to be fixed by the branch committee.
Between a quarter and a third of mainline rail drivers - the precise numbers are disputed by the company and the branch - are involved in the dispute.
DART services would not be affected.
The dispute is over the company's refusal to negotiate with the ATGWU branch on the introduction next week of rosters to facilitate an expansion of rail services.
Iarnród Éireann recognises two unions as having negotiating rights for train drivers, SIPTU and the National Bus and Rail Union (NBRU).
Its stance is effectively backed by the Irish Congress of Trade Unions, which says SIPTU has an established "sphere of influence" in the sector.
ICTU officials are likely to be involved in behind-the-scenes attempts this week to resolve the inter-union issues involved.
An ICTU disputes committee, however, has already ruled that the ATGWU does not have the right to organise train drivers.
If strike notice is served today, Iarnród Éireann says it will take legal action to try to prevent the action from going ahead. This is likely to focus on whether the ballot was properly authorised by the ATGWU.
Mr John Keenan, the company's human resources manager, said the manner in which the ballot had been organised was "a key question" which the courts would be asked to consider.
Mr Ogle said there would be no question marks over the legality of the strike once notice was properly served on the company.
He said that his union had a number of innovative proposals which it was prepared to put to both the ICTU and SIPTU with a view to resolving the inter-union issues.
