Flubber, a rare hooded seal, should be en route to colder waters today, following its release on the Porcupine Bank yesterday by the Naval Service.
Though it was not technically a fishery arrest, the seal was detained on the LE Niamh for some hours as the ship steamed over 100 miles west of Loop Head.
The marine mammal had been handed over by the Irish Seal Sanctuary to the ship's captain, Lieut Cmdr Gerry O'Flynn on Monday night.
Flubber was rescued by Mr Kevin McCormick off the Wexford coastline three months ago and was taken for recuperation to the Irish Seal Sanctuary in north Co Dublin. The hooded seal normally lives in colder climes, and is only the second of its type to be found off this coastline.
The mammal made good progress at the sanctuary and weighed nearly 100 kg when he was hoisted over the side of the ship yesterday in a crate.
The return was part of an international exercise undertaken by seal sanctuaries across these latitudes from Central America across to North Africa.
Mr Sean Eviston, of the sanctuary, said an international scientific investigation was underway to return up to 100 stranded seals to their habitats.
Also yesterday, the LE Niamh was given the task of picking up sensors from data buoys sponsored by the Marine Institute which had been picked up by fishermen offshore.