It's not exactly megalithic, although it was built with lots of stone. It's not exactly Neolithic, belonging more to the Medieval period. But the Old Abbey, tucked away in the centre of Drogheda not far from the main street, is a real historical and archaeological treasure. Its proper title is the Abbey and Hospital of St. Mary d'Urso.
Thanks to the fabulous efforts of local volunteers John Bannon and Eugene Brannigan, this historic ecclesiastical site has been cleaned up recently and is now looking resplendent.
The hospital was founded by Ursus de Swemele in the early 13th century outside the then medieval west gate of Drogheda. The remnants, according to the Sites and Monuments Record, consist of part of the nave, chancel, the western gable of the north aisle, and the central bell-tower.
The abbey was constructed of "roughly coursed limestone blocks, greywacke and rubble". Hardly anything remains of the nave except for some remnants of the southern wall which has been partly rebuilt with brick.
The following is a description of the monument from Curious Ireland:
This is Drogheda’s oldest monastic site as is known as ‘The Old Abbey’ or the ‘Hospital of St. Mary d’Urso’. The official name is however ‘The Abbey of St. Mary d’Urso’. It was founded in 1206 A.D by the Norman, Ursus de Swemele and his wife as a hospital for the sick and infirm. A flood in 1330 did considerable damage to the structure however it was restored mainly thanks to the generosity of the Brandon family of Drogheda. In 1349 it was given a ‘Royal Grant of Privileges’ and was later taken over by the Augustinian Crutched Friars as a Friary. The Abbey was finally surrendered in 1543 by the last Prior, Richard Malone, to the Drogheda Corporation, who proceeded to dispose of the monastic properties by lease. All that remains of the Abbey is the central belfry tower, a Gothic archway, an east archway and and a gable wall to the west.
Below is a Google map showing the location of the "Old Abbey", as it is fondly referred to by Drogheda people.