Manannan is popularly the god of the sea. Like Poseidon in Greek myths. But in Ireland the sea is a by word for death and the character of Manannan might be based on a group of death deities.
Other characters linked to the sea are Donn the first man in Ireland to drown according to the book of invasions and a death god according to the Dinsenchas story about Bull Island in Kerry.
Ladru is the steersman of the ships in the Cessarian invasion in the book of invasions. He Ferries the dead in the Testament of Cathir Mar.
Both the dead and the fish are called the Cattle of Tethra. A fomorian king inn the book of invasions.
And the most famous and scary of all the death gods Dearg. The red man from the destruction of Da Derga/the Red mans hostel.