In Viking times, the Danes were called ‘DHOO-GAEL’, that is black foreigners, while the Norwegians were called ‘FIN-GAEL,’ fair strangers. It has usually been supposed that this was a distinction made on account of a difference in complexion and colour of the hair in the two races but the Danes were not dark but fair and ruddy, as were the Norwegians.
The reason is that the Danes were so called on account of the dark metal coats of mail they wore.
The personal names ‘Dougal’ and ‘Fingal’ arose from the distinction.
The people of Man and of the Isles in these times were said to belong to the ‘INNIS-GAEL’ or ‘GALL-GAEL,’ the name used exclusively to denote the race springing from inter-marriages between people of the two nations, after Man and the Isles became largely inhabited by the Norse.
(source: Island Heritage (1952) by William Cubbon; photo from the film The 13th Warrior)