Manannan is considered a Gatekeeper to the world of the spirit for many reasons. The Land of Promise, one version of the Celtic afterlife, is known as his home. In many myths he displays the ability to not only bring mortals to the island afterlife, but to send them back the world of men again.
When the time came for the Tuatha De Dannan to retreat from the steady approach of humanity, it was Manannan that provided them with the ability to slip behind the veil, into an etheric world that cannot be perceived by most people. He seems to have no trouble travelling through any of the realms, from physical to the astral.
In modern times, Manannan can help see the souls of the recently depart safely on their way, should you wish it.