The name MORGAN is so old that it probably originated from Celtic Mythology in which the mother goddesses were MORRIGAN, MACHA and BODB. They were also sisters and took on the form of crows sitting on the shoulder of Cu Chulainn. The surname "Crowe" could therefore have a possible link.
Morrigan was the great Celtic Queen, a goddess of fertility, introducer of fear and irrationality. But nevertheless worshipped by the Keltoi before Christianity.
There may be a link with the Celtic tribe of CORNOVII, who settled in Mid-Wales and in the far north of Scotland.
The name MERRIGAN still persists in Ireland. Many Morgans in Ireland will have achieved their names from an Anglicisation of O'MUIREAGAIN.
PELAGIUS was probably born c.370AD in Britain, his birth being asserted by Prosper; Gennadius; Marius Mercator; Orosius and St. Augustine. Tradition has it that he changed his name to MORGAN (Seaborne). Jerome more precisely called him a Scot, ie, an Irishman.
The name MORGUND (Morgand) appears among the Dal Riadans who moved from Northern Ireland to Scotland in the 6th century. There are references in the "Book of Deer" the earliest example of Gaelic in a Scottish manuscript, now held in Cambridge University Library.