Notes:

Information about James William McGill came to me first from Katrina McGill (*1). In her 1973 letter she refers to her husband Ward's grandfather; James W. McGill, as one of the children of James F McGill and Isabel McFarland, daughter of widow Ma rgery McFarland.
Katrina further states that the marraige of James W. McGill to Emily Rumsey in 1850 by Jacob McGranahan, took place in Espyville, Pennsylvania, and was witnessed by J. F. McGill, Isabel McGill and Harvey Rumsey (Emily's brother). If this informati on is correct, and I have no primary evidence or vital records, then this would make a strong case for Isabel McFarland, born in Ireland, to have married a James F McGill.
Some good evidence is often found in the census records when the informent gives information regarding the birthplace of their parents. This was done first in 1870, somewhat, with the question regarding whether the parents were foreign-born. Howev er, the best information usually came first from the 1880 and 1900 censuses, when both parents' birthplaces were requested. This is probably about eighty per-cent reliable, since there could be errors from the informant-misleading or transcriber-i ntroducing the error.
The 1870 census for James McGill hasn't been located yet, and the 1880 census is very hard to read in the critical 'parents nativity' area. From what can be seen, it appears that James reports that his mother was born in Ohio, but it's very hard t o discern. The 1900 census finds James in Claridon, Ohio, where he reports, again, that his mother was born in Ohio.
If there are any other children of James F McGill, it might be useful to 'follow' them to see what they report for their mother Isabel's birth place.
(more here later)

Sources for James William McGill; Cabinet maker, House painter
1830 census; Poland, Ohio p 243 with his grandfather John McGill? (James' age 30-40)
1840 census; ?
1850 census; Poland, Ohio p 401 (his age 23, born in Ohio, carpenter)
1860 census; West Fallowfield, Crawford co, Pennsylvania p 1180 (his age 32, born in Ohio, cabinet maker, real val $100)
1870 census; ?
1880 census; Huntsburg, Guega co, Ohio ED 72 p 78 (his age 52, born in Ohio, house painter, f b in Ohio, m b in Ohio?)
1890 census; not extant
1900 census; Claridon, Geuga co, Ohio ED 47 p 72 (his age 72, born in Ohio, farmer, f b in Pa, m b in Ohio)

Other References
*1 Katrina McGill; 1973 personal letter to P F McFarlin