I wouldn't put much faith in the accuracy of Ancestry unless you know how the information was found. I'd buy the bapt and then also the marriage where you could first search for both names from that list of William Swans.

Good luck - hope it works out.

Just seen your second message. Did the Co. Meath site show you a transcript or the actual scan? A scan of the original would be best.
If you look at "about us" you can see that it is an Irish organisation, and local information is usually more accurate - just my opinion.

Sue