Hiya lads, this is by far my favourite mod for CK2 (well, tied with HIP) and I'm so excited for this to be completely polished when you're done! I have a few suggestions to add some historically accuracy flavour to Ireland, hopefully the first few are simple enough to add to CK2 before move on, but if some will screw with the balance or are just to complicated, certainly consider for CK3:
1) The king's steward was called 'Rechtaire' and a (major) minor title recognised Brehon Law would be a lawyer
2) After the Bronze Age, hillforts in Ireland not that popular. Ringforts (or ráth, in Irish) would have been the primary settlement of virtually all noble landowners right during this time. The most basic were univallate (single ditch) ráths , upgrading to a bivallate (double ditch) ráth, and finally a trivallate (triple) ráth reserved for the highest kings. After the triple ditch, it could then upgrade to a 'raised ráth' (even though these were technically univallate ringforts, raised ráths were often appropriated by the the Normans for their motte and baileys, so it'd make sense to require a raised ráth for a ruler to upgrade from tribal to feudal. Since that's just a name change to a building, hopefully this could be pretty easy to incorporate
3) The kingdoms in Ireland have elective gavelkind, where they should be essentially dynastic elective at least for the primary title (any male 7 generations out from the old king could inherit). Possibly allow for rulers one tier bellow to have a vote if ye want, since their interests would have probably have been taken into consideration as well? I don't know if other titles should just be distributed among the old kings kids, or if having an elector for each title would just be too hectic. From what I remember, Historical Immersion Project does a pretty good job with Irish succession law.
4) You have the Hill of Tara, but there were a few other royal sites as well. I get it would be a unbalanced to just blast Ireland with wonders, but for flavour, could ye add a few buildings with a little prestige bump? The kings of Connacht, Ulster, Leinster, and Munster would have their sovereignty publicly established at their respective hills, and if the petty king of the land surrounding the hill did not support the would-be highking, he could refuse him access to his property, severely damaging his carrier. So a bit of prestigious to whoever owns those titles:
Rathcroghan - Mag nAi province
Emain Macha - Ard Mhacha province
Dún Ailinne - Cill Dara
Hill of Uisneach - Uisneach province (mostly a pagan religious site)
Also, the Rock of Cashel, unlike Tara and the others, only really came into use as a royal site during the medieval period, so maybe have it unbuilt, or at least a first-tier building requiring an upgrade or two. The rest, being pagan, fell out of use during this time, so it wouldn't make sense to have them upgradable.
5) Finally, the fair of Tailtiu was an incredibly prestigious event for the kings of Ireland, held in Meath between July and August. It'd be cool to have a once in a lifetime option available to the King of Meath for a chunky bit of gold to declare a Fair of Tailtiu. It'd play out like your typical tournament event, but every tier king of Ireland would be eligible to attend. On top of that, the fair of Tailtiu was a way of declaring universal peace, so all attendees would have to end their wars in a white peace if they wanted to attend, giving the one who declares a Fair a 'enforce peace' option for the whole island!
Sorry for the fat post. If you could try incorporating at least the first few before the final update, I'd be eternally grateful. Thanks for all the hard work you put into this.