It's possible that William may have attended school at St. Edmund's Roman Catholic College in Hertfortshire: in the course of my research into the Hussey family from Kensington (members of which attended St. Edmund's and later married into William's family), I came across information on a William Butler who attended the college from 1880 until 1882 and who was born about 1870 in Brompton, Middlesex; our William above was born about 1870 in Brompton which makes me think they could be the same person.
Our William became a musical instrument maker in the family business. Sometime after 1902 (he was still living at home at the time of the death of his mother in July of that year) he moved to Dublin to run the family's Dublin shop, located by then in Monument House at 34 Bachelor's Walk. During the 1916 Rising, the shop was taken over by rebels (who emptied the building of its musical instruments, some of which were later found on the bed of the River Liffey at low tide). To oust the rebels, the British shelled the building from what was then The Red Bank Restaurant on D'Olier St. The shop was badly damaged and the business was relocated the following year to the ground floor of nearby 2 Lower Abbey St. where further troubles were to be encountered during the Civil War (1920-1923): on 6 February 1923 the IRA targeted the Pathé Frères cinema company which occupied the first and second floors of 2 Lower Abbey St. A number of armed men entered the premises, poured petrol around the Pathé Frères offices and set it alight. It's thought the men also planted an incendiary bomb as an explosion followed the fire causing a number of Pathé Frères employees to be thrown off their feet as they fled the burning building. Miraculously nobody was killed but the building was badly damaged. Although William did receive a small amount of compensation as a result of the Damage to Property Compensation Act of 1923, there was no proper insurance cover because the damage was caused by an act of war. As a result the company found itself in severe financial difficulties and was forced to close down in 1927.
William and his family, who had lived for a time at 2 Lower Abbey St. following the company's move there, later moved to Howth in Co. Dublin. William seems to have moved back to his old Abbey St. home sometime after 1923.
There are still Butler-made instruments in existence, both privately-owned and in museums. The Kenneth G. Fiske Musical Instrument Museum in Claremont, California, for example, has a Butler keyed bugle made in Dublin c. 1835. The Edinburgh University Collection of Historic Musical Instruments has a Butler flute and cornet (dates of manufacture unknown), and the Horniman Museum in London has a Butler harp acquired pre-1900, bright green in colour with Irish images, such as round towers and an Irish wolfhound, painted on the soundboard. And the National Museum of Ireland has a Butler bugle in its Easter Week collection. This bugle, manufactured c. 1915, has an interesting history in that it was awarded to the Irish Citizen Army in 1915 for taking first place in a drill competition; then, following the surrender of Irish forces (which included the Irish Citizen Army) in the 1916 Rising, it ended up in the hands of the British Provost Marshall who subsequently gave it to a Dr. Laurence Moran who in turn gave it to a brother of Fianna Fáil T.D., John McCann. John McCann's brother later presented it to Éamon de Valera, participant in the Rising and founder of Fianna Fáil, who would, in 1959, be elected President of Ireland. On 4 September 1948, Éamonn de Valera donated the bugle to the National Museum of Ireland.
Note: The Bachelor's Walk shop is mentioned in James Joyce's "Ulysses":
"From Butler's monument house corner he [Leopold Bloom] glanced along Bachelor's walk." (p. 151 of the Penguin Edition of 1960).