Scots Law recognised a number of forms of marriage ...
this is the modern interpretation
Irregular Marriages
The term ‘common - law’ husband or wife is often used but has no legal standing. However it is possible to be married without a civil, religious or humanist ceremony. This is known as an irregular marriage by ‘cohabitation with habit and repute’. To be recognised as married by cohabitation with habit and repute, the couple must:-
* live in Scotland
* behave as if they are married (not simply as lovers or cohabitees) and have produced a general belief that they are married (for example, call themselves by the same name)
* live together and have been doing so for a considerable time as husband and wife - a few years cohabitation is normally required
* not be married to someone else and must be free to marry each other. If they are not initially free to marry but become so and choose merely to live together, they may be considered as married by cohabitation with habit and repute. However, it is only the period when the couple have been free to marry that would be considered.
from advice guide scotland
That's all perfectly correct, but the marriage originally referred to above was not a marriage by 'cohabitation and repute', it was a marriage by declaration in the presence of two witnesses, and as such perfectly legal without needing to comply with all the conditions listed above.
however the church regarded any marriage without "benefit of clergy" as an irregular marriage
Any marriage that did not include a proclamation was considered an irregular marriage and was dealt with by the Kirk Session elders of the local parish. That could include eloping to a Justice of the Peace, or common law marriage. Elopement was a concern because of the not infrequent kidnapping of young heiresses.
from http://www.geocities.com/mjjodoin/irreg.htm
Again, that is all true, but by 1911 the Kirk had more or less had to give up trying to pursue folk for "irregular" marriages, especially in the cities. "Irregular" marriages were always legal, it was only the Kirk that took exception to folk escaping the 'discipline' of the Kirk.
Forfarian