I think you may find that your John TELFORD could be the son Richard TELFORD (1801-1888) and his wife Mary WHITAKER (1798-1866) of Tara, Offaly. This couple had a son Richard (1838-1923) who was born in Westmeath in 1838. Richard TELFORD's (1801-1888) father was John TELFORD (1756-1831) of Sallybrook, Clara, Offaly.
Captain (later Major) William TELFORD (1795-1873) of Derrycooley, Offaly and later Rathmines, Dublin and his brother John TELFORD (1802-1856) of Ballyboughlin, Offaly were two of nine children born to William TEFORD (1770-1837) and Eliza FINN (1765-1854) of Greenville, Clara, Offaly. This John TELFORD (1802-1856) did have a son John Gardener TELFORD (1842-2/12/1918) who died at West Derby, Lancs however it is my understanding that John G. TELFORD was born at Ballyboughlin and living in Toronto, Canada in 1881. He was certainly there in Dec 1880. His wife Eliza Jane BLUE died at Pine Bluff, Arkansas, USA in 1888.
Four of William TELFORD's (1795-1873) children migrated to Victoria, Australia in the 1850s. One of his sons, Charles TELFORD died at Derrycooley on 29/3/1860. William died at Rathmines, Dublin whilst living with his youngest sister Mrs Sally Elizabeth FLETCHER (1804-1878). Sally's husband was Dublin barrister, Wellesley Pole FLETCHER (1797-1869).
The TELFORDs arrived in Ireland around 1715 from Skipton, Yorkshire. John (1756-1831) and William TELFORD (1770-1837) were great grandsons of Michael TELFORD from Skipton.
Regards,
Self