That is because irish kings held mostly noble powers. They didnt have powers that the english kings had, their titles were mostly just for show. Remember that Ireland (like today) was never united and irish kings often battled over titles. The kingdoms (today are counties) were often subdivided into separate smaller kingdoms which fluctuated often, sometimes dissolving completely to be reformed into a smaller, or bigger, kingdom or combined with others. The four provinces today existed then with separate kings governing them. Some made it and some did not, but to answer your question, specifically with Boru, they had obviously more power than the local noble but governed their own provinces and were rarely regarded as a high king outside of them. They were being challenged by internal and external threats to their crowns consistently and Gaelic Kings of Ireland rarely held the crown for very long if at all. Hope this helped. I studied Irish History in college and that was a few years ago, if my info is off someone please advise!