Newcastle is a seaside resort historically in County Down in Northern Ireland; these counties have been abolished and it's now part of Newry, Mourne and Down District. The "new castle" was probably already tumbledown when first documented in 1433 and even its successors are long gone. Newcastle is best known nowadays as the home of Royal County Down Golf Club, and for its beach and wildlife reserve. It had a population of 7672 in 2011 and has lots of visitor amenities, so it's a good base for exploring the Mourne Mountains, which rise to the south.
The Visitor Information Centre on the Promenade is open M-F 09:00-17:00.