Willie Penrose has resigned as a Minister of State over the decision to close Columb Barracks in Mullingar.
The Labour TD had voiced strong opposition to the plan, which was approved at a Cabinet meeting this afternoon.
The Cabinet voted to close military barracks in Cavan, Clonmel, Castlebar and Mullingar. The Mullingar barracks will close at the end of March 2012.
Mr Penrose said he had tendered his resignation with regret.
"Over recent weeks and months, I had made my clear unstinting opposition to proposals to close Columb Barracks in Mullingar, so arising from a decision that was made at cabinet today, I had no alternative but to take this course of action."
"I was not prepared to stand over a decision that was not backed up by the facts and figures. While I will no longer hold the Labour whip, I will continue to serve the people of Longford Westmeath as an active and vocal public representative."
Tánaiste and Labour leader Eamon Gilmore said he regretted the decision - but said it did not surprise him.
Mr Gilmore said the decision to close the barracks was a difficult one, but "Government had to make difficult decisions in the interest of the country."
He described Willie Penrose as a good colleague who had worked hard for the party and his constituency.
The Tánaiste said the proposal to close barracks had been on the agenda for months and added that the issue of Mr Penrose resigning the party whip "does not arise at present."
Four military barracks to close
There are around 500 soldiers affected by the decision to close the four barracks in Mullingar, Clonmel, Cavan and Castlebar.
They will be relocated to other barracks.
The 170 members of the 4th Field Artillery based at Columb Barracks Mullingar will now move to Athlone.
The Mullingar Chamber of Commerce has said that the decision will cost the town between €8m and €10m in lost revenue.
200 soldiers are based at Kickham Barracks Clonmel. Part of the 12th Infantry Battalion located there will join the rest of the Battalion in Limerick.
Staff attached to the Headquarters of Defence Forces Reserve in Clonmel are likely to move to the Curragh, while soldiers with the headquarters of the Military Police will probably move to the Curragh or to Newbridge.
The 130 members of 6th Infantry Battalion based at Dun Ui Neill, Cavan will join the rest of the Battalion in Athlone.
The Reserve Defence Forces in Castlebar is also closing. This will affect 15 members of the Permanent Defence Forces - it is not yet clear where they will be relocated.
The overall strength of the Defence Forces has been reduced in recent decades, partly due to the peace dividend from the Northern Ireland peace process.
At the height of the Troubles in 1981 there were over 15,000 serving troops. That figure declined gradually to 10,500 in 2000. Now there are about 9,500 - the lowest since 1972.
There will be 14 permanently occupied military posts when the above closures go ahead.
Three of them are in Dublin - Rathmines, Blackhorse Avenue and St Bricin's Hospital.
The other installations are on the Curragh, Kilkenny, Cork, Athlone, Galway, Limerick, Dundalk, Gormanston (Meath), Finner (Donegal), Haulboline (Cork) and Baldonnell (Co Dublin).
Ten barracks have already shut down.
In 1998 the closures were announced of posts in Naas, Kildare town, Fermoy, Clancy Dublin, Ballincollig and Castleblaney.
Nearly three years ago four more barracks were closed - Donegal (both Rockhill Letterkenny and Lifford), Longford and Monaghan. St Bricin's Hospital in Dublin was to have closed but it remains open.
Dublin has seen three barracks close in recent decades - Clancy, Collins and Griffith.
Defence Forces Headquarters has partially decentralised to Newbridge.
There had been speculation that Cathal Brugha in Rathmines would also close, leaving just one - McKee Bks - open. But the Rathmines barracks escaped the axe.
Proceeds of sale of barracks have gone into the re-equipment and modernisation of remaining Army buildings.
The Defence Forces are now said to be more streamlined and efficient.