Llanfair Caereinion is in Montgomeryshire, Wales (not Merioneth) and is considered the cradle of the Jehu family, of whom there were (and are) very many in Wales and from whom I myself am descended on my maternal side.
I have a number of Mary Jehus in my family tree, for example
Mary Jehu, daughter of Hugh Jehu (1623-1690), my 8x great-grandfather. I have no dates or further info. on her.
Mary Jehu, daughter of Hugh Jehu (1690-1746), born 1725 in Llanfair.
However, I don't immediately see where your Mary fits in, although she is clearly somewhere in the family!I know that a number of Jehus emigrated from Wales to the US in the nineteenth century.
There is a lot of info. about the Jehus on Ancestry.com and Ancestry co.uk including family trees, the family having been researched back to Jehu ap Richard (c.1590-1646) of Llanfair Caereinion (and possibly earlier). He appears to have been my 11x great-grandfather!
Kenneth John Jehu Milton