By Ian Graham

BELFAST (Reuters) - An Irish nationalist acquitted earlier this year of the murder of two British soldiers in Northern Ireland was arrested on Friday in connection with the fatal shooting of a prison officer the previous day, a security source said.

The murder of David Black, 52, was the first killing of a Northern Irish prison officer in almost 20 years and the fifth fatal attack on a member of the security establishment since the 1998 Good Friday peace deal, which largely ended three decades of sectarian conflict in Northern Ireland.

Black, a father of two, was shot while driving his car in County Armagh. He worked at the top security Maghaberry Jail where militant nationalists have been protesting against their living conditions.

Two men have now been arrested over his shooting, the security source said.

Colin Duffy, 44, was picked up at his home in an early morning swoop in Lurgan, County Armagh, the source said, while a second 31-year-old man had been detained in Lurgan, where there is a strong pocket of militant nationalists.

Life-long Irish nationalist Duffy was acquitted in January of the murder of two British soldiers shot dead outside Massereene army barracks in Antrim by the Real IRA in 2009.

His co-accused Brian Shivers was jailed for life but Duffy successfully argued that police had planted his DNA, which was found in the car used by the killers. Duffy had been acquitted of murdering members of the security forces twice before.

Irish nationalists want the British army to leave Northern Ireland and for the province to unite with the Republic of Ireland.

Black, who was approaching retirement, was killed in a hail of bullets after a car pulled alongside him as he drove to work.

The vehicle, found later, was registered across the border in Dublin, Ireland. Prime Minister Enda Kenny has pledged the Irish authorities will do all they can to help in his murder investigation.

Kenny met Northern Ireland First and Deputy First Ministers, Democratic Unionist Peter Robinson and Sinn Fein's Martin McGuinness, for a North-South Ministerial Council in Armagh on Friday at which the murder dominated proceedings.

(Reporting by Ian Graham; Writing by Lorraine Turner; Editing by Andrew Osborn)