Dominic Hannigan, a Labour politician, was one of three TDs elected to the seat of Meath East this weekend.
The former civil engineer was elected as a Seanad Éireann senator on the Industrial and Commercial panel in 2007.
In 2006, while a county councillor, he marched with his partner in the Dublin Pride march.
Ireland has had openly gay senators in the past and may have its first gay president this year, if polls prove correct.
Senator David Norris, 66, is an independent senator known for his campaigning on gay rights. Recent polls have put him at the front of the race to succeed current president Mary MacAleese in the autumn.