I am posting this as would be interested to hear views from other family historians. The marriage took place at St Lukes Church of Ireland and some years back I downloaded the certificate I think from the Irish Family History Foundation website or similar. I suspect this certificate was the original. The groom William MCathie was shown as "widower", full age, cabinetmaker, son of George, a painter. To my surprise I now find a copy of the same record on irishgenealogy.ie website and there the groom is "bachelor" - everything else matches.
I suspect the groom may have been married 1stly on 5 Dec 1862 Dublin Castle, to Isabella Johnston, however I have not as yet gone for that certificate as my interest is in the 1872 marriage. I can find no record of a death of Isabella McCathie (first wife) 1869-1872 Dublin or elsewhere but not surprising as 4 children of the first marriage were registered under variations of the McCathie Surname. I do have details of births & baptisms but nothing thereafter. There were twins born in 1869 (McKetey) one of whom was "Isabella".
William and Margaret Critchley had 3 children from the 1872 marriage but I can find no record of birth of daughter Isabella thought to have been born abt 1873 in Dublin. The other two children were born in Scotland (William's birthplace) and in Queensland, Australia. William McCathie died in 1879 in Queensland. Leaving the possibility of a first marriage out of the equation how should I view the two 1872 marriage certificates as to Groom's marital status?!! I have put this in the "too hard basket" Many thanks Alma