Gardaí are continuing to question a man over the murder of IRA informer Denis Donaldson.
Mr Donaldson was shot dead at a remote cottage in Donegal in April 2006.
Just months earlier the former Sinn Féin official publicly admitted being a British agent inside the republican movement for more than 20 years.
Gardaí arrested a suspect in his 40s in Clontibret, Co Monaghan, yesterday evening.
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This is the third arrest in relation to the investigation. He is being questioned at Monaghan Garda Station where he can be held for up to three days under Section 30 of the Offences against the State Act, 1939.
PA