Robert, thank you for all the census info! I'm in US and can't easily search UK census except at the local Family History Center, so you're very kind to post all these details.
I haven't yet followed Mary's brothers forward, but it's a wonderful idea and next week I plan to do that. And of course I want to try to trace the Shehen/Shehans back to their Irish origins.
Yesterday I received the marriage cert for Mary Shehan [spelled Shehen] and John Slatter. This Mary is the 3-year-old in the 1841 census. She married John Slatter in Dec 1859, and the cert says that her father John Shehen is a bricklayer. Their daughter Mary was my husband's grandmother. I don't know when or where Mary Shehen Slatter died, but that's also on my list to find out.