Probably it referred to the shape of his tonsure.
So they brought him to Patrick, who entreated him gently and hospitably, and to Patrick he told the story of all that had befallen him.

But Patrick bade his scribes write all carefully down, that the memory of the heroes whom Oisin had known, and of the joyous and free life they had led in the woods and glens and wild places of Erinn, should never be forgotten among men.

And Oisin, during the short span of life that yet remained to him, told to Patrick many tales of the Fianna and their deeds, but of the three hundred years that he had spent with Niam in the Land of Youth he rarely spoke, for they seemed to him but as a vision or a dream of the night, set between a sunny and a rainy day.
THE HISTORY OF KING CORMAC CHAPTER XVI I THE BIRTH OF CORMAC Of all the kings that ruled over Ireland, none had a better and more loyal servant than was Finn mac Cumhal, and of all the captains and counsellors of kings none ever served a more glorious and a nobler monarch than did Finn, for the time that he served Cormac, son of Art, son of Conn of the Hundred Battles.

At the time at which this monarch lived and reigned, the mist of sixteen centuries hangs between us and the history of Ireland, but through this mist there shine a few great and sunlike figures whose glory cannot be altogether hidden, and of these figures Cormac is the greatest and the brightest.

Much that is told about him may be true, and much is certainly fable, but the fables themselves are a witness to his greatness; they are like forms seen in the mist when a great light is shining behind it, and we cannot always say when we are looking at the true light and when at the reflected glory.
The birth of Cormac was on this wise.