But the legislators of Rathbreasail did not entirely ignore tribal boundaries. On the contrary, so far as the numerical basis of their scheme permitted, they took them into account. And here we find that the Synod was confronted with another difficulty. The territories of tribes were fluctuating quantities. Hence, even if a diocese was the district of a single tribe, with very definite boundaries, no one could be sure that in the course of years its limits would not change. Again I take an example from Ulster. The Synod selected the Carntougher Mountains as the boundary between the dioceses of Derry and Connor. And wisely. For between those mountains and the Bann there dwelt a sept--the Fir Li--whose affinities were altogether with the people to the east of the river. But only a few years after the Synod that territory was overrun by the O'Kanes of the Roe Valley, and the Fir Li retreated across the Bann, never to return. The result followed which might have been expected. Their territory was transferred from Connor to Derry, and the Bann to this day is the boundary of the two dioceses.[55] It may be well, before I pass to another subject, to call attention to some special features of the Rathbreasail canons. First, let us note the prominence which is given to Limerick, the diocese of Gilbert, the president of the Synod. Usually a diocese is somewhat vaguely defined by four places on its borders. But here no less than thirteen are named. So full are the indications that a fairly exact map of the diocese could be drawn. Further, in this diocese alone mention is made of a Cathedral Church: "The Church of Mary in Limerick is its principal church."[56] Note the present tense: "The Church of Mary _is_"--not shall be--"its principal church." We remember that Gilbert insisted in the _De Statu Ecclesiae_ that a diocese should have a "pontifical church." Again, the boundaries of this one diocese are protected by a clause which has no parallel elsewhere: "Whosoever shall go against these boundaries goes against the Lord, and against Peter the Apostle, and St. Patrick and his coarb and the Christian Church." Who but the legate of the Pope would have thus invoked St. Peter? Surely this portion of the ordinances of the Synod must have been penned by Gilbert himself. And the whole passage--by the minuteness of its description of the diocese, by the strength of the terms in which it is expressed, by the reference to the Cathedral Church as already existing--suggests that the diocese was formed and organized before the Synod met, as I have already assumed. We may even suspect that an attempt had been made to invade it, which Gilbert stoutly resisted, relying on his legatine authority.