The names 'Alexander' and 'Young' together were not uncommon in the Londonderry area so it may be hard for folks to help you tie down your chap. Do you have any other bits of info on him at all?
On both IGI and another index, I can see births for an Alexander in 1870 and 1891 but nothing for 1880's (albeit the indexes are not complete) and there seems to have been about 7 Alexander Young's in Londonderry at 1901 Census. Also, at least four or five Alexander Young's married between 1909 and 1924, so any of them could have been in right timescale for the birth of William.