Population wise, Ireland has always had a larger population than Scotland. At many points, the Irish people swelled only to be killed off by the rather frequent disasters and famines that befell the island. Before the Irish Confederate wars and the Cromwellean Invasion; the population of Ireland was reckoned to have almost reached a parity with England. It's estimated that at least half the population died or starved between 1640 and 1650.
And all Scotland has to represent the complicated and intricate relationship between Highlanders and Lowlanders is an event giving you free troops against the English. The Irish did have a unique Feudal-Tribal system but so did many groups which have been drastically simplified. A few more unique national ideas might be interesting but I don't expect them to go into the minutae of Irish feudal politics any more than every detail of Manchurian or Italian politics.
Currently, the provincial make up of Ireland just doesn't make sense. There was hardly an independent King of Leinster, Munster, Connacht or Ulster. Those provinces consisted of loose alliances between Gaelic Lords and Anglo-Norman Earls. Some of these Anglo-Norman (Old English) lords were extremely powerful and up until Tudor times, they were largely autonomous. They married and integrated into Irish affairs. They even adopted the Irish custom of cattle raiding.
Not entirely true. There were most definitely Kings of Leinster, Munster, Connacht and Ulster, of varying degrees of independence. The amount of control exercised by this Kings was certainly less than those more centralized Monarchies and Princes in Continental Europe at the time, but as stated above I don't expect that level of detail. Otherwise, it'd almost be worth it to add a few centuries onto CKII with a mod.
Wikipedia gives me a useful list of Kings here;
https://en.wikipedia.org/wiki/List_of_kings_of_Connacht
https://en.wikipedia.org/wiki/List_of_kings_of_Leinster
https://en.wikipedia.org/wiki/List_of_kings_of_Ulster
https://en.wikipedia.org/wiki/List_of_kings_of_Connacht
There were certainly other petty Kingdoms, but I think the representation as 4 provinces is at least understandable. Again; there are more detailed mods out there and if Paradox added new provinces for Ireland, I wouldn't complain at all but I think what we have with this is pretty fair.
Certainly these weren't the centralized provinces we now know with clear boundries that EU draws for us, but nowhere in the world was like that until (arguably) the treaty of Westphalia, hardly at the outset of things.
In the realms of alternative history, if Ireland was to centralise at any point; it would more likely to have been at the hands of an Anglo-Norman lord than a native Irish King. Or at the hands of an Irish Lord that was savvy enough to adopt to English custom and tactics (Hugh "Red" O'Neill).
Culturally, Irish doesn't belong in the British culture group. England tried on many occasions to 'culture convert' Ireland. They only succeeded in one province (Ulster) but that place is more Scottish influenced than English.
The first count is debatable since EU and Paradox as a whole has always meant ridiculous history (Iroquois stomping Europe, Manchuria ignoring the Ming and colonizing America, etc) and further, my argument is that by the start point of EU, the line between Gaelic and Hiberno-Norman Lord was very, very blurred.
To the Second; I agree. I'm really annoyed at the loss of Breton as a Celtic culture. Most of my EU and CK play-throughs are Celtic games. That said, if Brittany is French and the entirety of Scotland is British, then it is no longer strictly a linguistic matter. In that sense, the Western Irish resemble the Highland Scots and the Hiberno-Normans aren't unlike the Lowlanders eligible to be recognized more generally as "British". Personally, I don't like it but that's what MEIOU and Taxes is for. Apparently they even have Norse-Gaels there.
If Ireland has to be grouped with someone, it should be the Highland Scots who are currently listed a British Culture. I would be really annoyed about this but I can understand why it would happen. If the Celts were given their actual historic boundaries (even if only Highlander Scots and Lower Bretons received the culture mark, though I'd be happy to argue in depth that Lowland Scottish and Upper Breton national identity as far back as the 14th century rested significantly on their Celtic heritage)
Yes. Same thing happens with Pommeranian becoming a german subculture despite whatever happened in your CK2 game.
That's unfortunate, though I suppose there might be a work around if I make all of my Breton provinces Welsh since the two cultures merging wouldn't be particularly out of character considering the way the timeline went. It's still a change I kind of mourn though.
Edit, responding to a couple of early points:
True, I'm all for the creation of Scots and Cornish as cultures. Though, I still think they should be included in the British Culture Group (and Irish, for that matter), because I doubt they'd have revolted if Scotland formed (or forms, in game) Great Britain.
Actually, Highland-Lowland animosity was definitely a thing. John of Islay and James III of Scotland marks a serious low point in this. If Scotland anglicized during the formation of Britain, then the Highlanders would definitely have been agitated. If they didn't, then the English would have been agitated and they would have broken their union with the Stewards far earlier, likely.
On the fence about Wales, since it does have a Celtic culture, but was almost completely subservient to the English by 1444.
Wales didn't have many open rebellions, in part due to the War of the Roses, where they believed the Tudors represented British ascendancy but they were slow to adapt to English institutions and certainly could have been made to rebel in different circumstances. Being an (initially) accepted Culture by the English does a decent job representing this.