Previous baptisms etc would be at Malew (about 2 miles away) - St Mary's was a chapel of ease; such chapels in general were only for Sunday services not baptisms and marriages (and as there is no graveyard except at Malew no burials except the very occasional intramural internment).
See http://www.ee.surrey.ac.uk/Contrib/manx/parishes/ca/casltwn.htm