Complex sentence: It is likely that an earlier church was on the site because when the floor was being relaid during the 19th century, fragments of pre-Conquest crosses were found, later shown to be "hiberno Norse" or Viking Age in origin.
Split into simpler sentences: It is likely that an earlier church was on the site because when the floor was being relaid during the 19th century, fragments of pre-Conquest crosses were found. These were later shown to be "hiberno Norse" or Viking Age in origin.