Nominations to fill two vacancies caused by the death of Tony Kett and the election of Labour's Alan Kelly to the EU parliament closed at midday today.
Only members of the Dáil and Seanad can vote in the elections.
Both FG and Labour put alternative candidates forward.
Because Mr Ó'Brolcháin and Mr Carroll were nominated by the Government side, their election is seen as a formality.
The results will be announced on 14 December.
A third vacancy in the the Upper House will be filled in the coming months.
