I assume the families were RC. If so, then sadly there are no parish records for Rasharkin before 1848. I can see from trees on Ancestry that one child John was born in Ireland in 1849. I can’t see that baptism in the Rasharkin records.
A portion of the 1851 census for Rasharkin has survived but I don’t see either the McKeown or McGuigan families in it.
The Ancestry trees also show a child James born in Kilbirnie in 1855. 1855 is a very good year for births in Scotland. The certificates for that year only contained a lot of additional information including the parents dates of birth and places of birth, the number of children they had had, alive & dead, as well as the date of the parents marriage. You might want to try and find that birth on Scotlandspeople.
Griffiths Valuation for 1862 lists 3 McKeown households in Rasharkin.
https://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
James in Glenbuck, Thomas in Killydonnelly & Arthur in Tamlaght. If you get no other leads you might want to look at those families to see if there’s any connection. No Brennan or Brannan households in the 1851 census fragments. No Brannans in Griffiths for the parish but there are several Brennans. Again you might want to look at them.
If the families were labourers they can be tricky to trace in the first half of the 1800s. They tended to move around a lot (to follow the available work) and so they may have lived in several different townlands. The general lack of records doesn't help either.
Or try the DNA route perhaps? Family Tree DNA reportedly has more people with Ulster roots than any other company. That obviously increases the chances of finding a match. You might want to try them or, if you have already tested, you can transfer your results to them for no fee.
The North of Ireland Family History Society is running an Ulster DNA project in conjunction with FTDNA and can offer testing kits at a reduced price. http://www.nifhs.org (Go to DNA project on the website).
