Pete,
I had a look at the Tithe Applotment Books at the National Archives. There are 32 entries for "Stones" in Co. Offaly. One is for Lurgan: Widow Margaret Stones seems to have had the largest plot of land in the village in 1827. Maybe she's the matriarch of your line?
For Fearboy, though, there are no Stones listed. So the Bridget Stones who died there in 1876 at age 77 (thanks for the info) didn't live there in 1823 - 1837, or remarried after her first husband died, or etc.. I read once you're lucky to get back to 1800 in doing Irish family history, and I'm beginning to understand why.
Mike