Guesstimate for her birth is seen as 1782 - it was probably more like 1772 based on her marriage record.
Married name was Fisher according to John Irwin's will. There was a Tobias Fisher enumerated next to John Irwin in 1790 in Cumberland Co, PA. I found no Mary Fisher anywhere near the age she must have been in 1850 in Clermont Co; she probably did not live until 1850.
In fact on Google Books, I found the Record of Pennsylvania Marriages prior to 1810, Vol. 1, by John B. Linn, William H. Egle
Census Records | Vital Records | Family Trees & Communities | Immigration Records | Military Records Directories & Member Lists | Family & Local Histories | Newspapers & Periodicals | Court, Land & Probate | Finding Aids