This is an excerpt of the first verse of "Rath Cruachan":

Listen, ye warriors about Cruachu!
with its barrow for every noble couple:
O host whence springs lasting fame of laws!
O royal line of the men of Connacht!

"Rath Cruachan" is poem 63 of The Metrical Dindshenchas (Lore of Places), a fabulous onomastic anthology written in Irish bardic verse which names the legends of signifcant places.

See "Rath Cruachan" at the Corpus of Electronic Texts site.