The head of the development board of the National Children's Hospital cannot guarantee that construction work will be finished by October 2024.
The original "substantial completion" target was last November.
Asked at an Oireachtas committee to guarantee that the revised deadline would be met, David Gunning replied: "I can't".
"We have got assurances from (construction firm) BAM they believe they can hit the October date," Mr Gunning said, but added: "I think the optimism has been beaten out of all of us in relation to this project."
He told the Public Accounts Committee (PAC) that "there remain challenges in getting this across the line" by next October.
Even if that target is met, the committee heard that the hospital will not open until "summer 2025".
Eilis Hardiman, Chief Executive of Children's Health Ireland (CHI), said that the "commissioning period" would take at least six, and possibly nine, months.
When PAC chair, Sinn Féin TD Brian Stanley, suggested that early 2026 might be a more plausible date for the hospital opening its doors, Mr Gunning accepted that this was "a possibility".
'Momentum is building,' he said.
Almost a third of the rooms in the hospital have been completed, Mr Gunning revealed, telling the committee that "32% of rooms have been offered".
In July, PAC heard that just 27 of the expected 3,000 rooms had been completed, which Mr Gunning today noted he had been "very disappointed with".
"So there's been significant improvement," he said, adding that "[we] are seeing some encouraging signs" and that "momentum is building in this particular area".
But Mr Gunning warned that the €71m remaining of the project's €1.43bn capital budget will run out at the "end of the year".
Derek Tierney from the Department of Health expressed his hope that the Cabinet will get an update on total costings by early November.
When Labour TD Alan Kelly asked those present if they disagreed with his suggestion that the ultimate bill would be "substantially more than €2bn", he was met with silence.
