Two men have been arrested in connection with the murder of Denis Donaldson, a former British secret service agent shot dead in 2006, Irish police said on Wednesday.

Donaldson was a former senior party official in Sinn Fein, formerly the political wing of the IRA, who in the months before his murder was outed as having spent 20 years spying on the movement for British intelligence.

Investigators did not name those arrested but said one is in his 40s and the other in his 70s. They were remanded in custody on Wednesday and can be held for up to 72 hours, police said.

Detectives appealed for anyone with information about the case to come forward.

"Both men were arrested in the Donegal area on July 26, 2016, and are currently being detained at Letterkenny police station," police told AFP by email.

Donaldson's body was found in April 2006 in the home where he lived alone, without water or electricity, in the rural village of Glenties in north west Ireland.