On the plus side, 48 year old Susan Jane (Patterson) McKenney's birthplace of Co. Kerry, so far away from her residence in Co. Louth in the 1901 census, is very positive, given the Kerry context for William's 1850 birth. Susan's reported age at death in 1909 was 62 (usual caveat about inaccuracy applies), for now it looks like a birth year in the region of the late 1840s - early 1850s.
It would just have been more conclusive to see a mention of William in the wills (a smoking gun if his full name of William Stewart had been used), however it may simply be the case that William was off the radar for the other siblings, perhaps he had made a life remotely from them for some time, after all, he was on the other side of the country on Achill Island in 1901.
In terms of the timeline for Robert Patterson, there is an indication that he was still in Castlegregory towards the end of 1857, a letter in the Cork Examiner of 14 October 1857 from Hugh Cooper, Chief Boatman at the Coast Guard Station at Castlegregory, mentions a Robert Patterson as 'one of the coast guard men' (Robert also gets an earlier mention in a list of Castlegregory Station men cited in the Tralee Chronicle of 19 January 1855). Since the 1864 newspaper article reporting his move to Dunany Point indicated that he was at Carlingford 'for many years', it would suggest that any move from Castlegregory to Carlingford came sooner than later after the 1857 newspaper reference for him.