"Esan a-rithist!"

Translation: Him again!

[dhoogl]

Why not "he again"?

[AlanS181824]

Esan (eisean in Irish) is the emphatic form of e/é meaning he/him/it. "He again" doesn't make sense in English so it's him again

[dhoogl]

"He again" doesn't make sense in English so it's him again
don't agree, but if esan would mean him rather than he, fair enough.

[FiferWD]

So would you say, "Him is here again"?

[PatriciaJH]

No; "him again" is short for "it's him again." The objective case "him" indicates that there is an implied subject and verb.