A man is being questioned at Letterkenny Garda station after he was arrested for allegedly being a member of an unlawful organisation.
The man is understood to be in his 30s and from Northern Ireland.
He was arrested in East Donegal yesterday but Gardai have not identified the specific location.
He was arrested under Section 30 of the Offences Against The State Act, 1939.
He was taken to Letterkenny Garda station where he is still being quizzed by detectives.
A Garda spokesperson confirmed “The investigation is ongoing No further information is available at this time.”