Voters in Dublin Midwest and Dublin Fingal will take to the polls this morning.
They will decide who will fill the seats left vacant in the Dáil by Clare Daly and Frances Fitzgerald, who were elected to the European Parliament in May.
While there are also by-elections taking place in Cork North Central and Wexford today.
Polls open at 7am and close at 10pm.
The last by-election was in 2015 and these are the first in the lifetime of this Dáil.
The usual election rules apply with voters asked to bring ID and their voting cards. Selfies and other photos in the polling centres are not allowed.
It's expected we will know the results of the by-elections tomorrow evening.
Many candidates are expecting the turnout to be low, with some saying they would be surprised if it topped 35 per cent.
The new TDs will take their seats in the Dáil for the first time next Tuesday.