Joe O’Brien is the Green Party's newest TD after topping the poll in Dublin Fingal.
Mr O'Brien received 12,315 votes after the eighth count in the constituency's by-election.
Fianna Fail’s Lorraine Clifford Lee came in second with 7,754 votes.
Duncan Smith of Labour was eliminated after the seventh count into third place.
Gemma O’Doherty was eliminated after the third count.
Turnout in the constituency was a new record low of 25.59%.
Mr O’Brien will now replace Clare Daly as TD for the constituency after she was elected as a member of the European Parliament in May.
He was the fourth and final candidate to be elected this evening.
Sinn Féin's Mark Ward won in Dublin Mid-West and two Fianna Fáil candidates, Malcolm Byrne and Padraig O'Sullivan, were elected in Wexford and Cork North-Central respectively.
Mr O'Brien's said today's result was more than just a ‘Green wave’.
He said: “Electoral politics can be deceiving, one person stands in front of the media like this but there was a big team behind me, we had a lot of people out on the grounds, we had a lot of people in HQ, my family in the background as well.
“It was more than just a wave, there was some serious hard work being done and a lot of people convinced on the doors as well.
Green Party chairman Roderic O'Gorman said the result indicated that the momentum gathered in May had carried on.