The Ulster units are available in any faction marked with (Ulaid) after their name. This includes units such as Ulster Spearmen, Ulster Kerns and the Royal Branch Guards etc. Yet if you play as either the Cenél Cónaill or Cenél nEógain who are both part of (Ailech) they can also have some Ulster units, as well as the two factions under (Airgíalla). They on their own have a few other unique units specific to their over-kingdoms but take some units from the Ulster roster.
The Connacht units are available with the faction marked (Connachta) after their name. This includes units like Connacht Spearmen, Cattle Raiders and Gamanrad etc. Yet if you play as the Uí Briúin of Bréifne they in fact get both Connacht, Ulster and some Uí Néill units due to their position and relations with the three over-kingdoms surrounding them, which gives them a massive advantage. The Uí Maine as well although are techinically under Connacht use units affiliated with the Shannon factions, such as the Uí Failge and the Déise Tuaisceart.
Munster, marked with (Mumhan) after the faction name, is in fact divided into two main rosters being the Eóganachta and the Déisi:
Specifiy mainly with cavalry and have Mounted Bodyguards unlike the rest of Ireland who use Arras Noble Swordsmen. These factions also can use Déisi Kerns as vassal units (recruited using the vassal building), due to the Déisi Muman and Deisi Tuaisceart being under Munster.
Have the best Kerns and really effective Swordsmen. Being vassals to the kings of Munster, their downfall is that their kerns are easily recruited by the Eóganachta and so they are hard to contend with. But the Déisi Tuaisceart lie on the Shannon, which gives them a slight advantage of recruiting Shannon units as well and indeed Connacht Mercs in the North.
Other Munster Factions:
The Uí Fidgenti, Uí Failge and Osraige usually are more affiliated with the Eóganachta roster but indeed have some of their own units, Osraige taking from the Leinster roster and the Uí Failge having some Shannon units.
Marked with (Laigin) after the faction name, the Leinster roster comprises of a nice variety of units such as Leinster Clan Warriors (spearmen), Dumnonian Archers (from Brythonic influence), and the Ríglach as Noble Swordsmen.
The Southern Uí Neill comprise of the two factions Mide and Brega who are the most powerful Kingdoms in Ireland, due to Brega being the High-King and Mide being a contender to the high-kingship. The Northern Uí Néill or (Ailech) comprise of the Cenél Cónaill and the Cenél nEógain, which with other Uí Néill factions have a roster comprising of Uí Néill Clan Warriors and Uí Néill Champions. Brega specifically get Teamhair Archers who guard the High-King.
Obviously Dyfed has its own unit roster, but this faction will NOT be playable for the sole reason that it is meant to be an exterior threat to the east coast of Ireland etc. Dál Riata also have a unique roster comprising of Dalriadan specific units and Pictish Units too.
In terms of Irish Factions I am unsure whether I will add more - this will be after I release a demo version anyway, But what I defininately plan to do is to try and add Northumbria (due to their relations with the Irish Royalty and their incursions in Brega) and possibly add Strathclyde or the Picts - but this is all undecided and won't be confirmed until after the first release of the mod.
Thanks for Reading, I hope this outlined briefly the faction categories I have implemented into this mod!