The Death-song of Corroi, Son of Dayry.
The Book of Taliesin XLII.
From The Four Ancient Books of Wales

THY large fountain fills the river,
Thy coming will make thy value of little worth,
The death-song of Corroy agitates me.
If the warrior will allure, rough his temper.
And his evil was greater than its renown was great,
To seize the son of Dayry, lord of the southern sea,
Celebrated was his praise before she was entrusted to him.

Thy large fountain fills the stream.
Thy coming will cause saddling without haste,
The death-song of Corroi is with me now,
If (the warrior) will allure.

Thy large fountain fills the deep.
Thy arrows traverse the strand, not frowning or depressed.
The warrior conquers, great his rank of soldiers,
And after penetrating enters towns
And . . . the pure stream was promptly whitened.
Whilst the victorious one in the morning heaps carnage;
Tales will be known to me from sky to earth,
Of the contention of Corroi and Cocholyn,
Numerous their tumults about their borders,
Springs the chief o'er the surrounding mead of the somewhat gentle wood.
A Caer there was, love-diffusing, not paling, not trembling. 
Happy is he whose soul is rewarded.

This poem, interestingly enough, refers specifically to the Irish story "The Tragic Death of Cu-Roi mac Dairi," from the Ulster Cycle.  The "Cocholyn" who causes Corroi's death is the hero Cu Chulain/Cuchulainn.  The "she" referred to in the first stanza is Blathnat, the wife of Cu-Roi, who was carried off by him, but was the lover of Cuchulainn.  In some ways, the story--particularly the name of the woman--parallels the story of Lleu Llaw Gyffes in The Mabinogi, with Lleu as Cu-Roi, Cuchulainn as Gronw, and Blathnat as Bloddeuwed (both their names mean "flowerface" or something similar).

The poem also shows the extent of Irish influence--particularly that of Ulster--on Welsh literature and myth by the time of the eleventh or twelfth century; the other prominent example of direct influence (as opposed to similarities of theme) is that of "Culhwch and Olwen," wherein Irish heroes of the Ulster cycle are listed among Arthur's warriors, and in the story of "Branwen uerch Llyr," in which the iron house of "The Intoxication of the Ulstermen" is used.