The move makes him the second Green Party TD after Neasa Hourigan to indicate they will vote for an Opposition motion, putting their futures as Coalition deputies in doubt.
"Having given much thought on the issue I can confirm I will be voting in favour of the National Maternity Hospital motion this evening. With the ongoing concerns from the public, constituents, and supporters, I cannot in good conscience vote against this motion," Mr Costello said.
"I understand being a TD in a government party and voting against the whip is a serious matter. I have informed my party whip of my intentions and I will accept the consequences from my action."
On Tuesday the Cabinet signed off on the building of the new NMH on land owned by St Vincent’s, despite concerns over religious influence on procedures such as abortion and sterilisation.
The Sinn Féin motion calls for a public hospital to be built on public land.
The Cabinet had agreed not to oppose the motion seemingly avoiding a divisive vote, but the Rural Independents called a vote during a debate on the Sinn Féin motion on Tuesday evening.
The Taoiseach today said tonight’s motion is all about playing politics.
Mr Martin told the Dáil that he “didn’t come down in the last cloud,” and has been “around the place long enough” to know that the Sinn Féin motion is intended to embarrass the Government.
The Government is not opposing the motion, meaning the disciplinary consequences for the Green Party pair expected to vote with Sinn Féin remain uncertain.
Sinn Féin said today that not opposing the motion meant the Dáil’s will would be that the land for the hospital should be owned by the State.
“We have a motion coming before the Dáil this evening. The motion is crystal clear,” said party leader Mary Lou McDonald.
“It sets out very clearly a call from the Dáil for full public ownership of the land. And by that I mean a clean transfer of ownership in perpetuity.”
She said she understood the “you do not propose to vote against” her party’s motion.
“So therefore I can only surmise that you accept that the best outcome is full ownership.
“I'm asking you as Taoiseach, given that you will support the motion or at least you don't oppose it, that you act on the will of the Dáil.
Engage with St Vincent’s (Healthcare Group) to secure the ambition of that motion.”
But Mr Martin replied: “I understand the politics of the motion. I've been around for a bit, so I understand Private Member's motions and the motivation behind them.
“It’s to put the Government under pressure. I understand that fully and so, you know, we will deal with the motion.
“You put forward motions with a very clear political agenda behind them.”
He said he was “fully comfortable with what's happening this evening,” but the important point was that the fundamental issue of ownership has been “dealt with.”
There was public ownership in terms of the 300-year lease, he insisted.