Ischia is a town and one of the six comuni on Ischia island, in the Tyrrhenian Sea. With a population of about 18,000, it is the largest se…Ischia is a town and one of the six comuni on Ischia island, in the Tyrrhenian Sea. With a population of about 18,000, it is the largest settlement on the island. Administratively, it is part of the Metropolitan city of Naples, in the Campania region of Southern Italy.