Thanks again to the kind soul who shared Walter’s and Bridget’s marriage record, this time for sharing Margaret’s birth record. This clears up everything and is pretty convincing evidence that both her grave marker and the transcript at Pat Deese’s website are incorrect. Margaret was born 12 Jan 1890. I think this means (and I hope to be able to eventually find the actual parish record to prove it) that Pat Deese’s date is a baptismal date. This makes me suspect that all the birth dates on her site are actually baptismal dates.
Also? Margaret and I share a birthday. I had no idea.