Just an update on what I have found since my first post:
William who died in 1909 actually did get his age right on one official document - the 1901 census where he got his childrens' ages wrong!
I have located his baptism in 1st Newtownards Prebyterian and he was born on 15 July 1848. His parents were William Blakely and Ursilla McCully (McAuley) and he had 3 other siblings in the church records - Anne Jane (1850), Ellen (1861) and James Alexander (1869). Anne Jane married James Quinn in 1871 (also in 1st Newtownards).
The elder William was the son of John Blakely and Anne Jane (Nancy) Kilpatrick of Greenwell St, Newtownards. His brothers and sisters are baptised in Newtownards Non Subscribing Prebyterian and include Ann Jane (m Samuel Morrow), James (m Jane Johnston), Robert and Charles (elder children predate church records).
I would love to hear from any descendants of this family and many thanks to everyone who helped me so far.