Mr William Luke Duffy was born in Castlebar, Co Mayo, Ireland on 8 October 1875 into a Roman Catholic family.
He was the son of John de Lacy Duffy, an engineer, and Ellen Ward who had married in Castlebar on 28 December 1870.
He had two known siblings, Patrick Joseph (b. 2 December 1873) and Mary (b. circa 1880).
Duffy received his education at St. Jarlath's College, Tuam, Co Galway. Upon completion of his schooling he spent two years as a clerk in Shackleton's Flour Milling Company, Dublin before joining James Walker & Co. of Dublin. He appears on the 1901 census of Ireland whilst he and his brother Patrick were living with their maternal aunt Mary Ward at 11 Stamer Street, Wood Quay, Dublin and William was described as a commercial clerk in a bakery.
William was married in St Andrews Church, Dublin on 13 July 1910 to Englishwoman Ethel Frazer (b. circa 1880); he was described as a clerk and his bride a nurse and his address was given as 95 (?) Lower Sagat Street, Dublin. Ethel hailed from Leeds, Yorkshire and was the daughter of Samuel Frazer. The couple settled in Southampton and had a daughter named Mary in early 1911; the small family appear together on the 1911 census of England living at Dean House, Millais Road, Itchen, Hampshire and William was described as a commercial traveller in the colour printing business.
The Titanic was William's first ship; when he signed on in Southampton on 6 April 1912, he gave his address as 1 Garton Road, Itchen. He had joined the Titanic in Belfast on 2 April and was on board for the delivery trip to Southampton. As the engineers' writer he could expect monthly wages of £6.
William Duffy was lost in the sinking and his body, if recovered, was never identified. His wife and child, and his aunt Mary back in Dublin, benefitted from the Titanic Relief Fund as class D dependents.