Thank you so much, Patricia. Your reply is greatly appreciated.
The 1900 US census shows that Mary (widowed) had 10 children, 8 living. Since Thomas and Mary wed in Feb 1851 and my great grandmother (Katy - the first child of theirs that I was aware of) wasn't born until 1854, I've wondered whether they had lost a child before her. However, of their 9 children accounted for, I've yet to find death information for two of them - Bridget b. 1864 and Julia b. 1868. I assume Julia died young because she's only listed in the 1870 US census, but Bridget was in both 1870 and 1880 censuses. Young, but conceivable old enough to have married and moved out.
Regarding the baptism of Thomas, I'm not sure about the date; but it seems his listed age varies from doc to doc. One thing, though, I know I have seen the name Dwyer before, but I cannot recall where. I searched a few times before replying, but came up empty handed. I'll keep looking, though. If you, or anyone else, have any suggestions about where I can search, please let me know.
Lowry