In 1048 some merchants built a hospital at Jerusalem for pilgrims to the Holy Land. It was dedicated to Saint John the Baptist and Pope Pascal II sanctioned the order under Raymund du Puy. Soon after the order became mainly military, and it was helped by the nobility. It was formed into a religious order with vows by Gerard who was the rector of the hospital. There are two modern associations which ascribe their origin to these knights, viz the Brandenburg and the English order of the Knights of Saint John. The latter founded the Ambulance system and the Red Cross society.
The house of the Knights hospitallers was founded here by Geoffrey De Marisco in 1215. St John of Alexandria was its first patron, and it was later deciated to St John the Baptist. Geoffrey was the kings lieutenant in Ireland for a long time.
The ruins of the abbey still remain. Its length is eighty six feet and its breadth is twenty two feet. The walls are over twenty feet high and five feet thick. There were five windows in the south wall and two in the north wall. The