I'm not sure about a full-blown HRE-style mechanic, the High Kingship was long dead by 1444 and was mostly ceremonial. I will be putting it in in some form though, and I definitely want to include alternative paths to unifying Ireland. I've got one idea for a chain of events and wars that I think will be fun.

I presume one of the paths would involve taking Dublin before taking any of other Irish minors causing nobles to defect in the minors who don't hold it, or just plain unrest for the people questioning their legitimacy.
Do you think you could adapt the centre of reformation conversion mechanic, In that lets say I own Dublin I can tick a button claiming Irish superiority that starts a foreign coring progress on 1 adjacent Irish province(since we don't want the effect to spread to england or scotland) or is that conversion mechanic built deeply into only the religion system.

Perhaps the Japan Dynamo mechanic might suit Ireland better with one Tuath 'elected' King.

Just saying that King in Gaeilge is Rí so implementing that would be cool

Why not create a japan like system, where dublin is a vassal of England and every irish minor has a war for high king cb? If they win they change tag to ireland and become englands vassal (bide their time), or they can declare independance.

Well why not a mission sort of thing to resurrect the O'Brien dynasty?