The certificate of registration of death for son John C. Dempsey lists father: unknown (illegitimate). Date of death is given as 16 October 1942, Jacquet River, Durham, Restigouche and date of birth as 12 May 1872.
John C.'s mother is given as Roseanne Dempsey, daughter of Patrick and Mary Reine, and is also listed as born in 1949. The Dempsey family of Patrick, Rane, Maryann, Anthony, Thomas, and Rose are enumerated at Durham, Restigouche, New Brunswick in Census 1851 when Rose is 2 years of age (born 1849).
Thus I would surmise it is a simple typo and the dates should be 1849.
Without having access to the evidence that supports that Stephen John Young is the father, I can only guess at possible matches such as the Stephen Young b. 25 Dec 1849 at Jacquet River; died 3 Nov 1935 at Richardsville, Dalhousie, New Brunswick; Husband of Marie Jeanne Ferlotte (1850-1937)