I've just had another look and there are 15 John Kellys born 1821-23 in 1841. Interestingly first I put in 1901 by mistake and found J.K. born Castletown, Ret'd Builder, living at 1 Rose Mount in Douglas with his family. Is this him and do you have those details? If not, I'll look again.

So, do you know what the names of John's parents were? Then I'll have another look. (by the way, There were 2 families in C'town with sons aged 8 and called John in 1841! )
Jo