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You would be right about Swalthy seeming unusual. It may be a less common spelling variant.
In the early 19th century, Roman Catholic Swalthy families were baptising children near the border between Co. Waterford & Co. Tipperary. See https://irelandxo.com/ireland/tipperary/clogheen-tipperary
On Christmas Day, 1899 a Mr & Mrs W. A. Swalthy were "among the latest departures from North Wall Station the London and North Western Company's Express passenger steamer" [Irish Independent] but they don't turn up on the 1901 Census.
We'd love if you added your Swalthy names above to their respective county Ancestors roll-call (to see what local volunteers may be able to match up).