I heard it used in, of all things, a discussion on Rugby, one of the panel an English pundit after referring to Ireland then said "This would also apply to the mainland" he was immediately confronted by the Irish pundit, who asked "which mainland are you referring to.".
As you say he was completely unaware that he had transgressed, but the term as used here by the Unionists is meant as a slight towards the Nationalists.

On collusion, I refer you to the Irelands Own Site, this is a comprehensive study of collusion. Ard Mhacha.