Annclare, I also found a baptism of an Eliza Hennessy, whose father was also named Thomas, on 9 Jul 1822 in Galbally and Aherlow parish (in Cashel and Emly diocese, same as the baptism you found) Tipperary, and her mother was Bridget Cleary. This baptism seems like it could be a little more likely just because it makes her at least 17 months older when she married and the mother's name is Bridget which is the name Elizabeth gave her only child. HOWEVER, I could not find baptisms (on Findmypast) of any other children of Thomas and Bridget Hennessy nee Cleary, so couldn't find a daughter named Johanna to link to the sister Johanna living with Elizabeth on the census, which makes the baptism you found more likely.