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 Stewart Frazer (born 1881); 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 (then a medical student, later a doctor) and his wife Mary Jane.

William and Ethel settled in Southampton and had a daughter named Mary Patrick, born 13 March 1911; the small family appear together on the 1911 census of England living in a 2 room apartment 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 (clerk) 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.

In 1921 Ethel was listed as living at Lydney, Kenilworth Road, Polygon, Southampton with her daughter and an unmarried aunt Marianna Ward. Possibly the same maternal aunt William had been living with in 1901.

In 1930 William and Ethel's daughter Mary married to Frederick Molony in 1930 and had a daughter Patricia Mary (1931-2019) and a son John Frederick Molony (1933-2015).  Mary died the following year at the age of 23. In 1939 John was shown living with his gandmonther in Eastleigh, Hampshire.

William's widow Ethel remarried to William Brooks in 1925.  She is believed to have died in 1940.