I haven't played the mod for a while so was pleasantly surprised when I noticed a Norse Gael culture (Gallawa) in the files. However they don't seem to be in any of the historical start dates. I don't know if they appear by event or aren't in game at all, but if you do intend to ever add them to the history files I have some suggestions to where they would best fit.
Depending on how you want to interpret the culture it may well be best to keep many if not all the early rulers of the isles as Norse/Norwegian so as they will align more with other Nordic rulers rather than Celtic ones, although if Somerled and his contemporaries are represented to any degree they would probably be better represented as a Norse-Gael, due to a greater connection to Scotland (this being more for gameplay reasons than necessarily implying a different culture).
Also whether represented as Gallawa, Norse or Norwegian, Gudrod Crovans vassals (at least the ones in Galloway and Mann) should probably be of the same culture. Currently the setup is such that a bunch of Norse-Gaels all hate each other because they think they are different cultures, which doesn't make much sense.
I also have some suggestions for the provinces near the isles/soreyar, whether you are adding the Norse gaels or not. Obviously these are just my opinions, but I feel that some of these changes could improve gameplay and historical immersion in the region.
If the Norse-Gaels were to be added to provinces I would suggest that at least Mann should be Gallawa from the old gods start, as while the Vikings did dominate the region they didn't completely oust the culture - the Manx people were a separate Goidelic peoples from the Scots and Gaels before the Norse arrived and remained so after they left and Norse Gael isn't a bad approximation of their culture. Currently ingame they change from Norse, to Norwegian to Scottish, which is a little jarring. Even if you aren't adding the Norse Gael culture may I suggest that you either leave the Manx in the north Germanic group for the whole timeline (ie Norse-Norwegian) or pick a Gaelic group to stick them in (Scots if you want them to integrate nicely into the kingdom of Scotland, Irish if you want them to be a bit rebellious). All of these groups could be argued to be a somewhat acceptable representation of the manx, but the current culture shift doesn't make all that much sense.
For Dublin and the Hebrides it could be argued that they should be Norse-Gaels from the tog start date but It would also be perfectly reasonable to have them as Norse at that point in time. However some point between 1000-1100 is the latest you could convert them to Gallawa as relatively shortly after that era the rulership of these areas passes out of Norse (or even Norse-Gaelic) rulership, and it wouldn't make much sense to have such a cultural conversion with rulers of a completely different culture.
As a final note, while most of the area of land represented by the province of Argyll was under Scottish rule rather than part of the kingdom of the isles for most of the game period, the game seems to use it mostly to represent the Island/coastal parts rather than the entire area. Because of this I think it would be better to represent it as either Norse or Gallawa in the earlier part of the game, and then either Gallawa or Scots later on, rather than the current setup up which has it as Scottish for the entire period.
Anyway, I hope at least some of that was of some help. Even if not thanks for keeping this great mod alive.