Portstewart is a small seaside town in County Londonderry, Northern Ireland. It had a population of 7,854 people in the 2021 United Kingd…Portstewart is a small seaside town in County Londonderry, Northern Ireland. It had a population of 7,854 people in the 2021 United Kingdom census. It is a seaside resort neighbouring Coleraine and Portrush in County Antrim. Its harbour and scenic coastal paths form an Atlantic promenade leading to a two-mile beach, popular with holidaymakers in summer and surfers year-round.