Minister for Defence Willie O'Dea said he was confident that St Bricin's military hospital would be used by the public health service.
He rejected as "rubbish" reports that the facility, which has a current 48-bed capacity, could not be used.
He also rejected Fine Gael allegations that his offer to the Department of Health was a "gimmick", and said that since he took office as Minister for Defence he had been "inundated with requests and people talking to me about St Bricin's in view of the accident and emergency crisis".
"We all agree there is a crisis. St Bricin's is a fine facility within easy distance of the Mater hospital, and people believe it should be made available."
An estimated €3 million would be needed for 25 more beds and an operating theatre. "That will be 25 more free beds in the Mater."
Pointing out that Minister for Health Mary Harney had spoken to him about St Bricin's two months ago, Mr O'Dea said "the Department of Health has not yet come back to me on the proposal. I believe the facilities can be used, and I am confident they will be."
He added that "all ministers can hold onto the empires in their departments. However, if one is lying on a hospital trolley different considerations apply."
Fine Gael's defence spokesman Billy Timmins described the Minister's proposal as a "gimmick", and said the hospital's director had told the Irish Medical Times the facility could not be used as a civilian facility in its current capacity.
However, Mr O'Dea said how much use the hospital got "will depend on how much money is spent. That is the point."
The director had said the hospital could not be used without expenditure being incurred, and "that is precisely what I have said all the time".
