If you google on "Dr. John Farrell" (using the quotes), you will come up with many pages to sift through, but could this be the John Farrell you are looking for? This may be drifting a little off topic for the list. If he is the right one, maybe you can work back from there. "November 5, 1887  Physicians Affidavit filed by Dr. J. H. FARRELL, who was a resident of Cambridge, Henry County, Illinois and had practiced medicine for 25 years. Dr. FARRELL stated that in 1877, JOHN L. BRYAN consulted with his father, Dr. John Farrell of Rock Island, Illinois. He diagnosed John with renal spinal and catarrhal disease and treated him for that problem until his death on January 22, 1884. The younger Dr. Farrell believed that reason for Johns death was caused by the original diagnosis. He stated that he also believed that undue exposure during his service in army was the cause of Johns disease.