It was common for Mass to be celebrated at the well on the saint's feast day.
Veneration of Holy Wells is just one of the many observances which at one time were very strong in rural Ireland. Others included "patterns", which was the practice of visiting a site associated with a saint, eg., a well, church or abbey ruin, etc., and there preforming an elaborate series of repetitive movements and prayers while walking around the site, sometimes involving moving stones from one place to another. This seems to have died out, and is looked on today a superstition but I'm sure it had something in connection with meditative prayer and was similar to medieval use of labyrinths. It is some years since I've lived in Ireland so I'm not really sure if the practice of venerating holy wells is still strong but I have been told that because of poor agricultural practice many of the wells are now unfit for human consumption and have health warnings erected nearby.