Pn the technical process, to amend the Constitution a Bill is initiated in Dáil Éireann and, once passed by both houses, the referendum is held. If it is approved, the President "duly" signs it into law. It doesn't go back to the Oireachtas as they have already passed it.

The only reason the President could refuse to sign it would be if he was not satisfied that the referendum operated in accordance with the law. If only there were a political party on the No side with enough links to organised criminals to run a ballot-stuffing operation. Even if they were caught, there'd be grounds for a re-run.