I took some time (and money!!) today and went through the Scots Origin OPRs. What a great time ... you almost forgot it's costing money!
Anyways, found the one and only OPR for Adam AIRTH chr 1755 ~ his parents are listed as Henry (Hendry) AIRTH & Isobell ALEXANDER marr Nov 24 1750 in Canongate Edinburgh Midlothian.
Further checking revealed only 1 Henry AIRTH that matched the time frame and he was chr Apr 11 1721 in Canongate to Alexander AIRTH (chr Dec 12 1700 in South Leith) & Anna GILCHRIST who were married Feb 11 1721.
Further checking led me to the parents of Alexander AIRTH ~ Alexander AIRTH & Margaret EDMOND (Edmound).I couldn't seem to find anything in the OPRs for a marriage between Alexander & Margaret but their children were all christened in South Leith Midlothian.