I was reading a facinating post by /u/legolodestone whom mentioned:
The Brehons were the true power in the land. They were a specialized druidic bard class that served as judge, jury, and lawyers. They had the power to over rule anyone at any point. When seated with the high King all the way through the 1300s they were served first and better cuts than the king. This wandering judiciaries would have been known by their outfits and often by face to all of the locals in the area. They ruled on all matters covered in every incarnation of the Kilkenny statutes (which is a whole lot) and anyone could request their services.
As I've never heard of these people before, I was wondering if anyone could elaborate more on who these people were, and, why the possessed such political clout?
Hiya! Well I really didn't think my side note would turn into a topic, but I guess fortune favors the brave. So here ya go.
The Brehons were a type of druid trained in name alone. They had absolutely no religious position and served as wandering lawyers throughout ALL of Ireland. They were trained under druids for the reasons of the druids having a) a collective influence b) the immense training that allowed for memorization of vast works.
The bardic aspect comes in as the Brehon laws, which morphed to suit the times in which they held sway over changing from pagan to Christian and changing to reflect those developments. Under these laws artistry was THE most important aspect as song was the primary method laws were preserved. This is the reason for the bardic aspect (the singing). Under the Uí Nialls the and later the Dal Riatans lore and law keepers became the primary connection to the other world via the ancestors and knowledge of the Tuatha de Dana (think a kingdom of mythic creatures who either fall in love with humans or think killing them is funny) and eventually as an insight to traditional values seen as being under threat. However this tradition bleeds through Scotland and into the Germanic settlements of England to be remarked on and used by the French in the great revival in the 1200s insert shameless plug for my advisor's book here.
This ultimately comes back to a respect for tradition. The Irish were so constantly at war that the Brehons served as a trained link to their past and served a pseudo "it was always done this way" reality. In truth the Kilkenny statutes and the Brehon laws evolved over a millennia until the 1300s. Initially they reflected a matriarchal society until morphing into the traditional westernesque values that they were conquered under all of which the Brehons memorized and could cite in cases of verdicts. Hence the value of seemingly impartial individuals who could practice without concern for their own well being. The lack of concern was because of how well treated they were, they never feared starvation etc. And because if their status as semi outside/above the system political reprisal wasn't really a threat.
It's worth noting at this point that traditional Irish culture emphasizes oral tradition as more true to the past than anything else. This stems from the belief that individuals can try to change the status quo but memory is untainted (this is untrue as the few annals show as compared to the statutes of Kilkenny). This results in the keepers if the ancient wisdom (the Brehons) serving as the voice to the past and being above question. It also ceremonially and practically placed their usefulness above Kings who originally we're only relevant in times of conflict.
SIDE NOTE(s)
Brehons laws, which Brehons ruled on in their journies, have pieces which some historians date back to Neolithic times and continue until the mid 1400s on various places.
The book is Reversing Babel by Bruce O'Brien
TLDR;
People respect tradition as a tie to their ancestors who must know better, so they are trusted over current rulers opinions.