Local lobby group Dublin Bay Watch claim that the 52-acre infill could cause a risk of flooding particularly for Clontarf but also in the port itself and in areas as far away as Sandymount.
Spokesman Liam O'Dwyer told the hearing that Dublin is a low-lying coastal city very seriously at risk from flooding.
He pointed out that the Environmental Protection Agency had recommended no further reclamation of estuary land in its report on the impact of climate change.
The Dublin Port Company has argued that a detailed environmental impact study had found that there was no risk of flooding from the proposed extension.
It has argued that the extension is vital for Dublin and the national economy as the port is nearing full capacity.
The company also argues that Dublin Port is at the heart of its market as 50% of all freight traffic services an area within the M50.
However, Dublin Bay Watch claim that future increases in freight traffic can be accommodated by greater efficiency at Dublin Port and by other ports on the eastern seaboard.
The group also says it would be premature to grant permission pending a report from the Government-appointed Dublin Bay Task Force and would conflict with a proposed special protection area order that would include the site.
