The four, all from Dublin, are the last IRA men imprisoned for weapons and explosive offences on the southern side of the Irish border to be released.

Prisoner release programme is key part of the Good Friday Agreement

Three of the men were convicted in 1996 for running a terrorist arms factory in County Laois.

Brian McNally, 57, who claimed to have been in charge of the IRA's arms manufacturing network, John Conaty, 37, and Gabriel Cleary, 54, were serving 20-year sentences.

They were thought to have been the IRA's main arms dealers and were caught in the act of preparing mortars and timing units for large bombs.

The fourth man being released, Dennis Lahiff, 31, was serving a nine-year sentence for arms possession.

The move leaves just 19 convicted IRA members in Irish Republic prisons, as well as some awaiting trials for alleged terrorist crimes.

Prisoner release continues

Around 50 IRA prisoners have been freed since the first IRA truce in 1994.

And 22 have been released since the Good Friday Agreement was struck by the UK and Irish governments and Northern Ireland's political leaders in April.

At least 400 paramiltary prisoners from the IRA, UVF and UDA are set to be freed under the release scheme, which is a key part of the accord.

Only those paramilitary groups that have declared and maintained a ceasefire are eligible to take part in the scheme.

The next batch of releases are expected to include men transferred from the UK to Irish jails over the past two years.