Taking some note of Falkyrn's posts on another forum:
A Clandestine Marriage was a criminal offence which carried a hefty penalty.
The Law Books of the period describe it as
1) a marriage conducted by a layman assuming the character of a clergyman or
2) a marriage conducted by a clergyman without banns or certificate of notice
....The definition of a Clandestine Marriage quoted is from the original statutes of the Scottish Parliament and much older than the book quoting them. (one book refers to it as a Common Law definition).
Certainly in the general sense a Clandestine Marriage was a form of irregular marriage but differing from the generally accepted forms in that it carried criminal penalties which could be dealt with away from the Church Courts. As you have said, the early Statutes listed differing levels of fines (in Scots Merks) for the Celebrator the participants and any witnesses.
One of the problems with early Kirk Session Records is that the terminology used depends very much on the individuals concerned - I have seen some records where the attitude was almost forgiving and showed a level of toleration I had not expected while in others the hellfire and brimstone almost leapt out of the pages. (my lot tended to favour that kind of preacher)
The fractious nature of the Church of Scotland did not help the situation and while referred to as the "Established" Church this status differs markedly from the way in which the Anglican Church is the Established Church of England.
Also:
...Clandestine Marriages and irregular marriages were very different under Scots Law and contrary to that article you have quoted the terms were not synonymous. The legal definition of a Clandestine Marriage is in my previous post and carryied hefty penalties for all involved - generally a 100 merks fine but a clergyman who performed such marriages in the early 1800's was sentenced to 3 months imprisonment and banished from Scotland for life. The confusion may have arisen because legally convicition of the crime could be used as evidence that the marriage existed but equally it could also be used to give either bride or groom an escape route by having the marriage set aside.
The definition of an irregular marriage varied over time - at one time it meant any marriage outwith the established Church but by the mid 1830's as other clergy were allowed to perform marriages the term irregular marriage came to be generally accepted as those marriages performed without benefit of clergy (ie non religious marriages). Such marriages did not carry a legal penalty but the Kirk Session took a pretty dim view of it and would call them to account for themselves.
Information from "A Handbook of Husband & Wife according to the Law of Scotland" by Frederick Walton BA Oxon, LLB Edin, Advocate published in 1893.
Monica