Fionn mac Cumhaill had a sword called, 'Mac a' Luin'. When it was made, a band of heroes had to take it to a special smithy to temper it. This band included the Oscar, Caoilte, and Daor-ghlas. The ballad tells what happened.
See: 'Folksongs and Folklore of South Uist' (M. F. Shaw ed., 1955) p. 29 'Leabhar na Fèinne' (J. F. Campbell ed., 1872) p. 59 and p. 65 'Stories from South Uist' (A. MacLellan & J. L.Campbell, 1961) p. 14 'Popular Tales of the West Highlands' (J. F. Campbell ed., 1861) Vol. III p. 396