Their 1827 Marriage Certificate says that "Christy McDond" was from "Drumin".
There is a website called
www.drumin.com about Drumin.
Drumin is a small settlement in the North East highlands of Scotland
It sits at the junction of Glenlivet and Strathavon.
Drumin lies in the district of Moray, although it formed part of Banffshire before the local authority reorganisations in the 1970's.
I wonder if this is any help in tracking down where Christina / Christy comes from.
The certificate says that John Buchanan comes from Eskadale.
http://www.visionofbritain.org.uk/place/place_page.jsp?p_id=22099 describes Eskadale.
Eskadale is a hamlet and a mansion in Kiltarlity parish, Inverness-shire, on the right bank of the river Beauly, 7 miles SW of Beauly town. The hamlet is small and rural, but contains a neat Roman Catholic church, St Mary's (1826; 600 sittings).
I wonder if John Buchanan's birth details are perhaps recorded in the old Parish Registers of the Roman Catholic Church, as on Scotland's People ?