Benny,
There was no “correct’ spelling of the surname. It will have varied all the time in Ireland. In a society where many people weren’t all that literate, it was down to the whim or knowledge of the priest or other person recording the information to decide how to spell it. But even in families that were literate, they didn’t seem to bother and so expect it to change all the time.
I assume your family was Roman Catholic. Statutory birth registration didn’t start in Ireland till 1864 (1855 in Scotland). Prior to that we have to rely on church records, where they exist. Unfortunately Rasharkin’s RC parish records don’t start till 1848, 2 years after your family left Ireland. So you are not going to find any marriage or baptism records for them, if that’s the parish they were born and married in.
You presumably know that Scottish birth certificates for the years 1855 and 1861 onwards normally record where and when the parents were married. And in 1855 only, additional information was noted about parents ages and places of birth. If any children were born in 1855, well worth viewing the certificate.
I have attached the tithe applotment records for Rasharkin in 1834. The tithes were a land tax so basically just list farmers, not labourers or servants etc. However if either of your 2 families were farmers then they should be there. (You should get Bernard & Mary Ann’s parents names & occupations from their death certificates).
There was a Magugan and a Maguigan in the parish. They are the nearest. (Magugan & Maguigan could be variants of McGuckian). No McKillops.
http://www.irishgenealogyhub.com/antrim/tithe-applotments/rasharkin-parish.php
Griffiths Valuation was compiled in 1861/2. It contains more of the population and there were 3 McKillop households in Rasharkin then (so possible parents, perhaps?) They were in the townlands of Carnfinton, Crushybracken and Anticur. If any of the names seem to fit, let me know and I’ll see if I can find a bit more on the relevant people.
http://www.irishgenealogyhub.com/antrim/griffiths-valuation/parish-of-rasharkin.php
Several McGuckians Michael in Duneany, Bernard, James jr, John Jr & John snr in Killycreen. So Killycreen might be a townland worth looking at.
1 Daniel McGuigan in Duneany in 1901:
http://www.census.nationalarchives.ie/pages/1901/Antrim/Glenbuck/Duneany/929466/
Denis McKillop in Anticur in 1901:
http://www.census.nationalarchives.ie/pages/1901/Antrim/Glenbuck/Anticur/929356/
Jane McGuckian in Anticur in 1901:
http://www.census.nationalarchives.ie/pages/1901/Antrim/Glenbuck/Anticur/929335/
The townlands I have mentioned are all very close to each other and I’d guess therefore that your family came from one of them. You can see their relative locations using the placenamesni site:
https://experience.arcgis.com/experience/9b31e0501b744154b4584b1dce1f85…
If you can provide some parents names and fathers occupations, I’ll search further.
A portion of the 1851 census for Rasharkin has survived but unfortunately it does not include any of the townlands mentioned so far except Carnfinton. 1 McGugan household:
http://www.census.nationalarchives.ie/pages/1851/Antrim/Kilconway/Rasharkin/Carnfinton/8/