Labour leader Eamon Gilmore has been elected on the first count in Dun Laoghaire with 11,468 votes.
The Fianna Fail candidates, Mary Hanafin and Barry Andrews, suffered badly, taking 5,090 and 3,542 votes respectively.
Based on tallies and the transfer of votes, the other seats are likely to go to Fine Gael's Sean Barrett and Mary Mitchell O`Connor, with Ms Hanafin struggling to take the last seat ahead of People Before Profit candidate Richard Boyd-Barrett.
Mr Gilmore said he was confident he could bring in his running mate, Ivana Bacik, in the constituency.
"It's an historic day for the Labour Party," he said.
"This is the first election in the history of the state that the Labour Party is going to emerge as the second largest."
Dr James Reilly, the deputy leader of Fine Gael, was elected on the first count in Dublin North.
Independent Shane Ross won a resounding victory in Dublin South, topping the poll with 17,075 and became the first non-party candidate to be returned.
Socialist Joe Higgins, a key player in the United Left Alliance, claimed its collection of left-wing candidates could build on his likely success in Dublin West and take four seats, or up to seven depending on how votes transfer.
His running mate, Clare Daly, looks course to take a seat in Dublin North.