A Green Party MEP does not believe building housing on reclaimed land in Dublin Bay is a viable option.
The President of the Royal Institute of Architects of Ireland, David Browne, said homes for 150,000 people could be constructed on Sandymount Strand and the Tolka Estuary.
A similar idea was proposed 100 years ago but it was shelved.
Speaking to the Irish Times, Mr Browne said he believes the expansion of Dublin needs to stop, and instead densify.
He said when the tide goes out there is massive areas of land that would be of benefit to the city.
He cited examples in Europe of where comparable ideas have worked, including in Copenhagen.
However, Dublin MEP Ciaran Cuffe thinks it could be damaging to the environment.
"I think it's very controversial," said Mr Cuffe. "Personally, I don't think it's viable.
"It involves building in the Dublin Bay biosphere which was a Unesco designation a few years ago."
Mr Cuffe said that now is not the time to do it, "because of the impact on nature, because of the threat of climate change, because of so many other factors."