Hi Angie
The Irish usually guessed at their ages and ages were often mis-judged by 10 years either side....I have just been looking at the BMD on ancestry and there are 2 possibilities for your John and your Patrick....Firstly, John Kilkenny born about 1842, Death July 1915, Bury Lancs.....Secondly Patrick Kilkenny, Born about 1866, Death June 1919 Bury, Lancs...
If this Patrick is the brother of John than he was definately born earlier than 1866...The informant of the Death either guessed the age or if it went to the Coroner then the Coroner would have performed an autopsy and summised the age too....This is exactly what happened with my own G G Grandfather (who was also Patrick Kilkenny) in Leeds in 1911.....The cornoner underestimated his age by a good 6-10 years.....