1) I find it odd that this family had a William Joseph Cahill, then a Joseph Cahill, do you? 2)In the 1910 NY census, @Children's Hospital, Randall's Island NY:
Cahill, Dennie age 14 b. NY Cahill, Joseph age 14 b. NY
Odd that their ages are the same, but the names fit. Perhaps Dennie was Francis' middle name? Nickname?
In 1930 Census, Queens NY: William J. Cahill 37 (b. 1893)occ: US Post Office Sophie A. Cahill 29 William S. Cahill 6 Gloria E. Cahill 4 10/12 (is this your William?)
WWII Draft Registration, 1942: Edward M. Cahill 549 Ishom St NY NY born May 21, 1890 wife: Mary Occ: US Post office @8th ave & 33nd St. (seems like I read that your gr-granddad worked for the Postal Dept also?
US Veterans Gravesites, Sophie Cahill BM 2C US NAVY (rank of husband) b. 20 July 1895, died 10 June 1992, Wife of Cahill, William Interment: 15 Jun3 1992 Long Island National Cemetery 2040 WEilwood Ave, Farmindale NY 11735 (if a wife is buried in a National Cem. the Serviceman HAS to be buried there also) Is this your Gr-Grandma??