"We went to Portmarnock and took the dolphin on board," said Colin Murray, of the Howth Coast Guard Unit.
He said a member of the public had been holding it in the water when the boat arrived.
"It was then dropped to Ireland's Eye, where the water is deeper," he said.
It's unclear how the dolphin came to be too close to the shore, in water too shallow for it to swim out again.
"The tide was going out, and there is a lot of sand in Portmarnock. It didn't look injured or in too much distress," he said.
After release, the dolphin swam away.