Bevin,
The townland is probably Tullymacnous (208 acres) which is in the parish of Killyleagh. No Cains or Irvine households listed there in Griffiths Valuation (1863).
http://www.askaboutireland.ie/griffith-valuation/index.xml
Killyleagh 1st Presbyterian baptism & marriage records start in 1693 so you may be able to trace the families in that. The records are probably not on-line and you would need to check in PRONI, Belfast to access them. (Personal visit required by you or someone on your behalf).
You could also check the tithe applotment records for 1820/1830 which may contain one or both families in the townland. Again the records for Down are not on-line and a personal visit to PRONI is required.
A little information on the townland in this link:
http://freepages.genealogy.rootsweb.ancestry.com/~rosdavies/PLACENAMES/…
Elwyn