DUBLIN, Ireland (CNN) -- Six Irish Republican Army prisoners, including one who was jailed for murder, were released late Friday from a high-security prison near Dublin as part of a Northern Ireland peace deal.

The release marked the first time prisoners have been freed under the Good Friday peace agreement, a deal reached in April that calls for the release of jailed members of the Catholic IRA and Protestant guerrilla groups within two years as long as the groups keep to the cease-fire.

Three of the prisoners released from the Portlaoise jail were recently transferred from prisons in Britain. They included Adrian Donnelly, who served nearly 21 years of a life sentence for murder; Michael O'Brien, who served six years of an 18-year term for the attempted murder of a British policeman; Vincent Wood, who served six years of a 17-year sentence for possession of explosives.

The other prisoners released were Frank Burke, sentenced to four years for IRA membership, Michael Cully, who served two years of a 12-year sentence for possession of guns and explosives, and Simon Maxwell, who served almost two years of a six-year sentence for possession of explosives.

Northern Ireland Minister Mo Mowlam said last week that prisoners from four guerrilla groups would be excluded from the early release program because Britain believed they were still "concerned in terrorism."

Britain has yet to announce the release of Northern Irish guerrilla prisons from its jails. But the British Broadcasting Corp. reported Saturday that Britain could start the release within the next few weeks and inmates could be leaving jail by September.

More than 200 republican and loyalist prisoners could be released by Christmas this year under the scheme.