Hello Michael,
I thought Anne/Anna Swan's age was given as 22 (daur of William Swan) in the marriage cert copy you found for Sept 1876? Which if accurate gives her birth date as between Sept 1853 and Sept 1854. And the census gave her birthplace as Meath. I therefore thought the baptism of Anne d/o William Swan in 1853 Co. Meath was pretty close! But of course needs checking.
Sue