In keeping with the Homeric analogue of Proteus, a god changing into animal shapes on the seashore, Stephen looks out on the ocean and thinks, "They are coming, waves. The whitemaned seahorses, champing, brightwindbridled, the steeds of Mananaan." Manannán Mac Lir is an Irish god of the sea who is sometimes depicted Poseidon-like driving a horsedrawn chariot (or riding a single horse named Enbarr) over the waves. Stephen thinks of him again in Scylla and Charybdis.
Manannán is strongly associated with the Isle of Man, which lies NE of Dublin in the Irish Sea. His Irish name comes from an old version of Man, and in many stories he provides protection and good fishing to the Manx people. It seems possible that Stephen is thinking of this connection as he looks out to the sea and sees the waves rolling in from the east.
In Scylla and Charybdis, George Russell's mention of King Lear appears to trigger Stephen's memory of two lines from Deirdre, a play by Russell first performed in 1902: "Flow over them with your waves and with your waters, Mananaan, / Mananaan MacLir ..." Gifford describes the "chant in Act III (p. 49) spoken by the druid Cathvah (a part played by Russell himself when the play was first performed). The chant calls down a druid curse, the Faed Fia (which Russell took to mean the last flood, the end of the heroic age), on the Red Branch Knights just before those heroes begin to quarrel among themselves and destroy a comradeship-in-arms comparable to that of the Arthurian Round Table." The druid's curse asks the god to destroy the company of knights.