Sheerness is a port town and civil parish beside the mouth of the River Medway on the north-west corner of the Isle of Sheppey in north Ken…Sheerness is a port town and civil parish beside the mouth of the River Medway on the north-west corner of the Isle of Sheppey in north Kent, England. With a population of 13,249, it is the second largest town on the island after the nearby town of Minster which has a population of 16,738.