The man arrested is in his late 60s and is understood to be from Belfast. He was arrested in the south-east of the country on Wednesday evening as part of a planned operation by gardaí.
He is being held at Killarney Garda Station under section 30 of the Offences Against the State Act 1939, which means he can be held for an initial period of up to three days.
A number of police forces, including those in the UK and Northern Ireland, are involved in the fresh investigation, which began earlier this year, into the disappearance of Mr Brooke Pickard, a father of four who had moved to live in south Kerry, but who was never seen again after he left to go turf-cutting on April 26, 1991.
He was aged 43 at the time of his disappearance.
An excavation took place in September, as part of the fresh investigation into his disappearance, at a remote An Coillte site on a mountain pass 20km from Waterville, near the spot where Mr Brooke Pickard’s van was found burnt-out.
Mr Brooke Pickard and his wife Penny moved to Ireland from England in the mid-1980s, buying a farm and old farmhouse.
He left to go turf-cutting on the morning of April 26, but Penny and the couple’s four children, aged 5, 7, 11, and 15, never saw him again.
Witnesses claim they saw Mr Brooke Pickard being abducted by up to five men close to his home.
Penny and their son Crohan, now a father of five, continue to live in south Kerry.