Laois man Eoin Barry had conceded defeat after the first count.
Disabilities campaigner Tom Clonan will replace Ivana Bacik in the Seanad.
He won the Trinity College by-election last night, after the 16th count.
The security analyst got 5,358 votes - beating Maureen Gaffney by 160.
Laois Labour Party rep Eoin Barry conceded defeat after receiving 189 votes (1.4% of the poll) in the first count.
The seat became vacant when Labour's Ivana Bacik won a Dáil seat last year.
Tom Clonan says he was shocked to win:
He will be the 27th senator to represent the university since 1938.
He says his son Eoghan will be his inspiration in the Seanad: