Manannán mac Lir had another name: Orbsen. From Orbsen, Lough Orbsen — now Lough Corrib in Galway — took its name, because when his grave was being dug the lake broke forth from the earth. The name corruption is traced by O'Flaherty: Orbsen became Oreb, then Orib, then Corrib.

At Magh Ullin — now Moycullin — Uillin, grandchild of Nuadh of the Silver Hand, "overthrew in battle, and had the killing of, Orbsen Mac Alloid, commonly called Mananan (the Mankish man), Mac Lir (son of the sea), for his skill in seafaring." O'Flaherty records this in his West Connaught, published by the Irish Archaeological Society in 1846. In the middle of the seventeenth century, the lake was still called Lough Orbsen.

Keating gives Manannán's genealogy through the name Orbsen: "Mananan, the son of Alladh, the son of Elathan, son of Dalboeth, an immediate descendant of Nemedius, the progenitor of the Tuatha de Danans in Ireland." The Tuatha Dé Danann, Keating continues, "are still believed to rule the spirit or fairy land of Erin; to reign paramount in the lis, the cave, the mine; to occupy genii palaces in the deepest recesses of the mountains, and under the deep water of our lakes."

Under the deep water of our lakes. The god of the sea lies beneath a lake, and the lake came from his grave. Gill noted the connection in 1929: wells and lakes as thresholds to the Otherworld, and Lough Corrib as one of them — "said to have issued from the burial-place of Manannan MacLir."