Two different St. Mary's, one RC and the other Church of Ireland, John baptised to Richard and Elinor 10th December 1758 on IrishGenealogy is Church of Ireland, so the NLI registers will not help
Pro Cathedral was not built until the early 1800s, so a different church was used before then for St. Mary's catholic parish.
p.s. It's a bad, slightly sideways '8'