The Metrical Dindshenchas is a series of ancient legends connected with the origin of Irish place-names. They come to us from the distant past and survived mainly by word of mouth over the centuries before being written down in manuscripts by Christian monks in medieval times. There are five volumes in all. Some of the material is presented here, selected because it relates to sites already featured in Mythical Ireland.
Ath Cliath Cualann (Dublin)
Hurdles of wattling the Leinstermen made in the reign of Mesgegra under the feet of the sheep of Athirne the Importunate when delivering them to Dun Etair at the place in which Allainn Etair was taken from Ulster's warriors, where also Mesdedad son of Amergin fell by the hand of Mesgegra king of Leinster. So from those hurdles "Ath Cliath" ("the Ford of Hurdles") was named.
Or thus: "Ath Cliath": When the men of Erin broke the limbs of the Matae, the monster that was slain on the Liacc Benn in the Brug of Mac Oc, they threw it limb by limb into the Boyne, and its shinbone (colptha) got to Inber Colptha ("the estuary of the Boyne"), whence "Inber Colptha" is said, and the hurdle (clíath) of its frame (i.e., its breast) went along the sea following the coast of Ireland until it reached yon ford (áth); whence "Ath Cliath" is said.
Source: Ancient Irish Tales, Tom P.Cross & Clark Harris Slover, 1996, Barnes and Noble
Tonn Clidna
Clidna daughter of Genann son of Tren went out of Tulach Da Roth ("Hill of Two Wheels"), out of the Pleasant Plain of the Land of Promise, with Iuchna Curly-locks to get to the Mac Oc. Iuchna practised guile upon her. He played music to her in the boat of bronze wherein she lay, so that she slept thereat, and then he turned her course back, so that she rounded Ireland southwards till she came to Clidna.
This is the time at which the illimitable seaburst arose and spread throughout the regions of the present world. Because there were at that season Erin's three great floods, namely, Clidna's flood and Ladru's and Baile's; but not in the same hour did they arise: Ladru's flood was the middle one. The flood pressed on aloft and divided throughout the land of Erin till it caught the boat and the damsel asleep in it on the beach. So there she was drowned, Clidna the Shapely, Genann's daughter, from whom "Tonn Clidna" ("Clidna's Wave") is named.