there was nothing "in concealment under ground in Erinn, or in the
various secret places belonging to Fians or to Fairies" that they did
not discover and appropriate. This statement receives strong
confirmation from a Scandinavian record, the _Landnáma-bok_, which
says[22] that, in or about the year 870, a well-known Norse chief named
Leif

"went on warfare in the west. He made war in Ireland, and there
found a large underground house; he went down into it, and it was
dark until light shone from a sword in the hand of a man. Leif
killed the man, and took the sword and much property.... He made
war widely in Ireland, and got much property. He took ten thralls."

Although the Scandinavian record does not speak of the owner of the
earth-house as either a "Fian" or a "Fairy," it is quite evident that
this is an example of the plundering referred to in the Irish chronicle,
and that the Gaels of Ireland seven or eight centuries ago, if not a
thousand years ago, regarded the underground people as indifferently
Fians and Fairies.[23]

Many other associations of Fians with Fairies are to be seen. In one of
the old traditional ballads regarding the Fians, they are described as
feasting with Fairies in one of their "hollow" mounds.[24] A
Sutherlandshire story relates the adventures of the son of a Fairy
woman, who took service with Ossian, the king of the Fians.[25] One of
the Fians (Caoilte) had a Fairy sweet-heart.[26] Another of them (Oscar)
has an interview with a washerwoman who is a Fairy.[27] A Fenian story
recounts how one day the Fians were working in the harvest-field, in the
Argyleshire island of Tiree, and on that occasion they had "left their
weapons of war in the armoury of the Fairy Hill of Caolas";[28] from