The thing about Irish high-kingship is that it was never an actual political office, and indeed no legal tract recognizes it as an institution with corresponding status (and the Irish were obsessed with status). Rather than being an actual political office like the King of France or England, the Irish high-kingship was the regal equivalent of a 'world's greatest dad' t-shirt.

When someone was eulogized as High King, it was merely a recognition that they were the most powerful king on the island who exercised political and military hegemony (but not institutional or bureaucratic control) over at least half of the island, if not more. Brian Boru was notable for being the first king to exert hegemony over more than half of the island, but he wasn't the first to be called "High-King". That distinction goes to Mael Sechnaill I, King of Tara who was eulogized as "King of all Ireland" upon his death in 862, nearly a century and a half before Boru's time.

As for Brian Boru, his success was not really due to anything extraordinary. He 'played the game' like all other Irish warlords who came before him by extracting submission from neighboring kingdoms through force or coercion. His successes were in fact due to a historical process that had been unfolding for centuries: the steady militarization of Irish society followed by the introduction of Scandinavian coastal towns which provided Irish kings with both Scandinavian warriors and a steady stream of wealth from their transnational trade networks. By Boru's time, Irish kings had access to economic and military resources that would have been unimaginable to earlier generations; had Brian Boru never been born some other regional king would have probably played the same role.