After searching unsuccessfully for many years as to the origin of the surname MCKIRGAN, I have rested on my theory that it is derived from The Old Norse language, specifically from the word "KIRKJA" meaning "church." and is pronounced "KIRKYA." The Icelandic version of the word is "KIRKJAN", pronounced "KIRKYAN."
Common knowledge among historians, is that the Scot and Irish prefixes MAC and MCK stands for "Son Of." Speaking with a Scot or Irish accent, one can hear how the name "MACKIRJAN" can be spelled phonetically, eventually settling on it's present spelling including it's variations.
Correspondence with the appropriate heraldic offices in Scotland, Northern Ireland and the Irish Republic all concluded that the surname "MCKIRGAN" did not appear to be indigenous to the British Isles.
The Norse invasion into those areas beginning around 793 AD introduced their culture to the local populace. Today we see the results of their influence in the English language, customs and cultures. There are many websites on the Internet that explain this in greater detail.
We may never know the exact location of where the surname derived, but in my opinion, it's Scandinavian in origin. I believe it's possible that our McKirgan ancestors were among the Norse who settled in Iceland, then moved into Scotland and finally into Northern Ireland. If there are other theories besides this one, please share it with us.