An LIRR conductor was assaulted by a sloshed rider after a raucous Hamptons St. Patrick’s Day party because MTA cops failed to police drunken passengers, according to a federal suit.

Luke Beharry, 31, was manning a boozy westbound train after the annual Montauk bash when a drunken reveler allegedly punched him in the head and gave him a concussion.

Beharry, who is seeking $1 million in damages, said MTA cops were nowhere in sight during the 2011 fracas.

His lawyer, Michael Flynn, said Beharry missed three months of work.

The MTA declined to comment.