Afraid you got it slightly wrong, davieboy! It doesn't state her parents married in 1834 - it actually states that Mary herself was born and registered Ailsa, Isle of Mull in September 1834 (may well have been born September but it was 1832!).
Her father was indeed deceased and formerly a farmer I thought (hard to read) but now I think it says pensioner.
Scotlands People has these baptisms:
Neil and Mary McKinnon, lawful twins, bp.11/10/1832 Kilninian & Kilmore, Argyllshire, dau. of Donald McKinnon and Rebecca McKinnon.
Noted some siblings:
Lachlan McKinnon, bp.31/7/1830 Kilninian & Kilmore, son of Donald McKinnon and Rebecca McKinnon.
Another set of twins:
Allan (must have died) and Sally McKinnon, lawful twins, bp.17/5/1835 (Donald shown as pensioner)
Allan McKinnon bp.8/2/1837
Cannot find the marriage of Donald McKinnon to Rebecca McKinnon which one would think to find late 1820's. However, looks like Rebecca died must have died 1837 as he swiftly appears to have remarried (with 3 children he'd have needed another wife - his youngest three all died I think):
Donald McKinnon, Pensioner, and Catherine McDonald were married 16/5/1837 Kilninian & Kilmore. No status given for any couples on the page.
1841 Census (per FreeCen): Abbass, Ulva Qs, Kilninian and Kilmore, Argyll
Donald McKinnon 65 Army Pensioner born Argyll
Catrine McKinnon 40 ditto
Catherine McKinnon 14 ditto
Lauchlan McKinnon 12 ditto
Niel McKinnon 8 ditto
Mary McKinnon 8 ditto
Spellings are as have been transcribed. Cannot find a baptism for Catherine McKinnon and wonder whether she might have been the daughter of second wife Catherine (??)
I decided to check for a military record for Donald McKinnon and think this could be him:
Donald McKinnon, private 92nd Highland Infantry, born Kilfinnichan, Argyllshire served 20years 207 (?) days having enlisted 25/5/1794 and discharged 15/12/1814 due to being worn out in the service, was wounded through the left ancle (as spelt) in Holland in October 1799 and the calf of the right leg in Egypt in ye.......(runs off the page). He was 42 when discharged (bc.1772 - this ties in with 1841 census which although states age as 65 (b.1776) aged were rounded down for the 1841 census). He was 5' 4.1/2", brown hair, hazle (as spelt) eyes, swarthy complexion. By trade a Labourer. (Army record is on FindMyPast).
Think Donald must have died between 1841 and 1851 as can't see him after 1841.
So Mary was a little older than she thought and was in fact a twin. Looks like the other twins and last child Allan must have died pre 1841.
Hope this helps.
Annette