Edward was 4 months. Thomas Cain was farming 60 acres of Ballachrink Farm (thus probably reasonably well off) - not sure what 69 Ballachrink means suspect you are confusing an entry number.

If William was 13 in 1881 then probably the John Thomas son of Thomas Cain + Eleanor Quirk bapt St Johns 28 JUL 1867 (can't see any other and names fit) however if he married in 1885 then age 18 is very young for a male marriage - have you checked the marriage certificate (ie buy it from the registry office) and checked the fathers name etc ?

A trip to the Manx Museum will give you immediate access to all censuses on microfilm (+ Brian's indices to help find entries) as well as the entry in the parish reg which should confirm an under age marriage - or join the IoM Fam Hist Soc and go to library in Peel on Tues or Wed afternoons