The primary purpose of the OPRs was to record baptisms, marriages and (if you are really, really lucky) burials carried out by a Church of Scotland minister in that parish. While there seems to have been some sort of intention that all births / baptisms and marriages should be recorded, in all my families (I'm 1/8 Scottish descent) I've only ever seen this happen for one family in one parish.
If I examine the page of marriages that you posted, virtually every entry records a non CofS minister - Burgher ministers, Relief ministers, presumably the Gaelic ministers as well. It's therefore quite clear that this page represents part of some sort of attempt to record all marriages in the parish. So it's perfectly possible that a Catholic marriage would be recorded there.
What I'm less certain of is the exact meaning of the phrase used for their entry. "St. Andrew's Session" will surely refer to the Kirk Session of a Church of Scotland church called St Andrew's. (Logically, you should check that one of that name exists in the area.) Not to a Catholic church. Irregular marriages were unions that had not taken place according to the rules of the CoS. Marriage By Repute is one example where the couple live together without a ceremony taking place but everyone actually believes that they are married. (Not the same as just living together).
Whether a Catholic ceremony would be rudely dismissed as Irregular just because it was Catholic, I have no idea. If you could get hold of the relevant Kirk Session records (Scotlands People is supposed to be releasing them sometime) then they might shed light on the reason, because I guess that they discussed the matter and had the marriage recorded in the OPR. The date is probably just the date that the Kirk Session discussed the matter or the OPR entry was made.
So the Session in question is probably just the body making the report of what it saw as an irregular marriage.
I take it that you've checked the RC records on Scotlands People?
According to this page, the Church of Scotland seems to have recorded all marriages.
Before the introduction of civil registration in 1855 Church of Scotland parish ministers and session clerks kept registers of births and baptisms, proclamations of banns and marriages, and deaths and burials.
Thus, I'd say yes to "Would a Catholic irregular session marriage at St. Andrews in Glasgow on 25/11/1821 be listed in the OPR?"