Cormac Ua Liatháin was a 6th-century Irish saint who is mentioned in Adomnan of Iona's biography of St Columba.
In Adomnan's narrative, Cormac gets mentioned three times. Cormac appeared to be a kind of anchorite monk who searched for islands on which to live as a hermit in prayer. It is also said in the narrative that he founded a monastery in Ireland.
In the first occasion, Columba used his clairvoyance to tell others that Cormac had just set sail 'from the district of Erris, beyond the river Moy‘ that day to find a place of retreat but he found none. Columba said that God would not allow him to find one because he was travelling with a monk who was away without his abbot's permission, although Cormac was unaware of this.
In the second occasion, Cormac made a second attempt to find a place of refuge by sailing off into the sea. Columba clairvoyantly and prophetically knew all about this, and at the time he was in Pictland with King Bridei I and Columba, knowing that Cormac would find his way to the Orkneys, which at the time was a sub-kingdom under the Picts, Columba therefore requested the King to make sure that if any of his brethren came to the Orkneys, that they would be treated well. Bridei obeyed the request and when Cormac actually arrived there, he was protected from harm. According to the narrative, a few months later the monks at Iona were talking about Cormac and whether his voyage had succeeded, and Columba told them that Cormac would arrive at Iona that day, and he then did arrive just as the saint foretold. It is not clear from the narrative whether this meant he only spent a few months in Orkney in total, or if he later went back to Orkney after visiting Iona.
According to the narrative, however, Cormac also made a third journey into the ocean to find a place of refuge. His ship was driven by a steady southerly wind for fourteen days which pushed him very far to the north. Then his boat was attacked by a large swarm of tiny monsters, that were frog-sized, but which had spines on them. Cormac feared they might puncture the skin covering of the boat and he prayed to God. Columba, in his clairvoyance, was aware of what was happening and he called his monks to come and pray together for Cormac. In answer to these prayers, God then reversed the wind and made it send them back to the south. Cormac then came to see Columba again.
In the third occasion, Cormac along with three other Irish saints (St Cainnech, St Comgall and St Brendan (moccu Altae)) came to visit Columba and they found him at the island of Hinba. They chose St Columba to be the celebrant at mass and during the mass Brendan saw a ball of radiant fire over Columba's head.