There was no notion of constitutional law in the modern sense. Early Christian Ireland was very much a fragmented society: there would have been more than 100 petty kingdoms, each with their own king, and then most of those kings would have recognised the overlordship of another greater king. Then there were five kings of provincial status: there seems to have been no real single king over the whole island, although a number of kings would have claimed this but in reality they would not have controlled the entire area.
Q: - So there was no concept of invasion of territory during the Anglo-Norman period, when the Anglo-Normans arrived? Did the Brehons not feel this was an intrusion on their national territory?
The Brehons would have had different ideas of national territory. As we know the Anglo Norman invasion was an extraordinarily successful phenomenon and this would partly have been that there wasn’t that very strong sense of national identity - obviously they would have known that they were Irish and spoke Irish - but in general the people would have identified with their own particular part of the country, rather than having had a sense of Irishness.