Robert McCann’s marriage to Mary Bonney was Roman Catholic so that rather confirms their denomination. The civil certificate shows that both were illiterate and signed with a cross. That may well account for the variations in the spelling of the surname that you have found. That wasn’t uncommon at that time. Nobody bothered much about having a “correct” spelling then anyway.
Robert was a bachelor, aged 27. His father was Thomas McCann, deceased. He had been a pensioner, so that indicates he had served in the army. Robert’s mother was Letitia Pearson and was alive in 1858. Mary Bonney was a widow, aged 28. Her parents were Thomas Bonney, labourer, deceased and Mary Murphy, alive. One of the witnesses was Patrick Bonnie (note spelling). Mary’s previous husband’s name is not given. I don’t see a likely previous marriage for Mary in Scotland.
I looked for marriages in Ireland for the 2 sets of parents but without success. A problem you face is that many RC parishes in Ireland don’t have records for the period you need ie marriages 1810 – 1830.
There is a tree on Ancestry (Steve Whitby family) with Robert & Mary. It records Mary Boney (spelling) as dying in Cumberland (England) in April 1870.
Robert & Mary are in the 1861 census for Whitehaven, Cumberland. His unusual occupation (bee hive maker) makes him fairly easy to identify. The family consists of Robert 28, Mary 30, Mary 7 & Thomas 2. The children were born in Scotland and the parents in Ireland.
Margaret McGowan also appears in that tree, born 15.3.1859 in Dumfries and dying 2nd May 1931 in Glasgow.
Many Irish labourers in south west Scotland came from Co Down. It’s only 20 miles across to Scotland and until about 1850 the main ferry service between that part of Ireland and Scotland operated from Donaghadee in Co Down to Portpatrick in Wigtownshire. So very easy for people to cross to that part of Scotland. So it wouldn’t be too surprising in the McCann/McKean family came from that county. Here’s a link here to a Co Down site with loads of McCanns:
http://freepages.rootsweb.com/~rosdavies/genealogy/SURNAMES/Mc/McCann.htm
There’s a Letitia Pearson on the site too, but she was Presbyterian and married someone else:
http://freepages.rootsweb.com/~rosdavies/genealogy/SURNAMES/P/Pe.htm