The seats up for grabs are in Dublin South-West and Roscommon-South Leitrim.
It follows the election to the European Parliament of Fine Gael's Brian Hayes and Independent Luke Ming Flanagan in May.
Legally the seats have to be filled within six months – but the government is expected to move the writs today, so that the two by-elections can be held on Friday the 10th of October – the Friday before the Budget.
