Update 4pm: A cabinet minister says she does not support the idea of a prayer before sittings of the Dáil.
Katherine Zappone says she would prefer if no religions were observed at the start of every day's business.
But she has not said if she will oppose the government tomorrow and vote in favour of a proposal to scrap the prayer entirely.
Michael Nugent of Atheist Ireland is encouraging opponents of the prayer to lobby their TDs ahead of the vote.
"We've already has on this island a Protestant parliament for Protestant people in Northern Ireland, we knew that was wrong.
"A Catholic parliament for a Catholic people would be equally wrong."
Update: 11.48am: Debate is continuing about rules surrounding the Dáil prayer.
TDs will vote tomorrow on whether they should have to stand for its duration.
If they don't, the new rules would state they could be kicked out of the chamber.
Solidarity TD Ruth Coppinger says the direction the rules are taking is baffling: "Why is the hierarchy in the Dáil trying to embed archaic practices at a time when society is moving in the opposite direction.
"At a time when people are completely questioning Catholic church control over our hospitals, over our schools, and yet the Dáil is making it a stipulation that wasn't there before."
Earlier: The Dáil will vote tomorrow on new rules that would oblige all TD's to stand up for the house's opening prayer.
It follows last night's debate on a proposed new format that also includes 30 seconds of silent reflection.
Some left wing TD's say having a Christian prayer no longer reflects a multi-cultural Ireland.
And a handful say they will break Dáil rules by refusing to stand.
But Independent TD Mattie McGrath says those opposing the format are out of step with public opinion: "I've yet to encounter any widespread resistance to the practice among the majority of members.
"It seems to be a neat preoccupation among those on the hard left.
"It has certainly found no traction among TDs and indeed the general public.
"Respect for other cultures or religious views should not ban us from acknowleging the specific heritage of our own country."