Hi everyone,
This is Antagonist from Twcenter (using my ancient Taleworlds account), Skandinav asked me if I could help out with research for this faction. I've been involved as a researcher, writer and tester on a number of TW and M&B mods, many set in the Viking era, and the Gaels are probably my specialist area. I don't claim to be a true expert (there are very few, the military history of this culture at this period is pretty understudied) but I think I'm reasonably well informed, at the least. It's been a while since I played Warband and the multiplayer game in particular, but I loaded up Vikingr the other day after Skandinav contacted me. I didn't do very well (to put it mildly) but I was very impressed with the quality of the project. It'd be a pleasure to help out.
Anyway, I thought I'd start out with a brief overview/definition, just so everyone's on the same page. In 1066, the Gaelic world essentially consists of two nations, Ireland and Scotland, as well as the various islands and so forth between them. Both are nominally ruled by a national monarch, but this position is quite tenuous in practice, with the king struggling to assert authority over powerful regional princes. Nevertheless, there is a surprising degree of cultural uniformity (i.e it's not a bunch of different clans/tribes all with different cultures, militaries etc.) The main exceptions are that some areas have absorbed more Norse influence than others, and Scotland has residual Pictish influences and cross-border English ones that Ireland does not.
1066 actually puts you after the nominal end of the "Viking Age" in both countries, as the powerful, independent Norse kingdoms and colonies such as Dublin no longer threaten to overrun or dominate, and are fact being slowly absorbed into the Gaelic polity. Nevertheless there was still low-level conflict, raiding and so on between Norse and Gaelic peoples throughout the 11th century, and several major flareups, notably during the reign of Magnus III of Norway. Magnus seems to have wanted to resurrect Norse dominion in the Irish Sea, and made a number of Gaelic expeditions, which culminated in his death in battle in Ireland. At the same time, forces from Ireland continued to raid and interfere in Wales and elsewhere on the west coast of Britain, while Scotland fought periodic border-conflicts against both the Saxons and Normans.
I guess I should write some general commentary on their military and on the considerable influence of Norse tactics and equipment, but I'm running on a little here. Basically if you want to preserve the Light/Regular/Elite/Leader structure, then Capn' Morgan's suggestions are certainly on the right track. I'd suggest a slightly different arrangement though. A quick overview:
Light: Ceithernn (Plural Ceitherne)
Later anglicized as "Kern", Ceitherne are essentially Gaelic levies and light troops. Very lightly armoured (if at all), they function primarily as skirmishers and flank troops. Their primary weapons would be throwing spears of various kinds (small darts, regular javelins and sometimes large, heavy throwing spears for breaking shields and piercing armour) with short spears, small axes and long knives, and sometimes small bucklers, in melee. Archers were historically almost never employed in Gaelic armies, with slingers being the preferred long-range weapon, but they did become more prevalent in the 11th century, especially in Scotland.
Regular: Cliathaire (Plural Cliathairi)
Mercenaries and other regular, professional soldiers. Forming the backbone of an army, these are career warriors who have accumulated decent military expeience, with the prestige and superior equipment that go along with that. Again, they would be armed with a throwing weapon (likely fewer, heavier throwing spears) and any number of possible melee weapons (spears, swords, axes etc.) They would probably be less heavily armoured on average than their Anglo-Saxon or Norse counterparts, but they would have some kind of padded coat or leather armour and helmet at the least, with the better-off having metal helmets and the very best a coat of mail. If possible, they should be infantry by default but have the option to fight mounted as light, skirmisher cavalry.
Elite: Gall-Gaedhil
There are several possibilities for this slot, such as Curadhi/Champions (basically Cliathairi who have acheived particular distinctiona and fame) and the bodyguards of lesser chiefs and princes, but I think these guys would be the most interesting. Gall-Gaedhil means "Foreign Gaels" or less literally "Norse-Gaels" and refers to warriors of mixed Gaelic and Norse decent, though their ranks also contained actual Norsemen fighting as mercs. Such people were viewed with some suspicion by the powers that be, but they had a formidable reputation and were frequently employed as mercenaries. Their equipment varied widely, but they tended to be more heavily equipped on average and to be placed in the vanguard of the battle-line, hence their elite status. Basically heavier, more offensively-orientated infantry with more Norse equipment (dane-axes and so on) With some custom artwork it would be possible to convey a really interesting "hybrid" look for them.
Leader: Toiseach
There are a number of possible terms for a mid-to-high-level battle leader on par with a Norman count or English earl. Both Toiseach and Mormaer are specifically Scottish terms, but the latter would be at least understood in Ireland as well. It more-or-less means "chieftan" or "leader" (it survives these days as the title of the Prime Minister of Ireland, actually) As with the other factions, the leader is very similar to the elite soldiers, but with the very best equipment. Mail, a fine helmet (often with a horse-hair crest), a heavy shield, javelins, and a sword or axe for melee. Again, if possible, they should be infantry that can elect to fight on horseback if desired.
Those are my initial suggestions. I'll try and work on getting more sources and images, but don't hold your breath for a wide array of illustrations. Osprey, for example, has little for the Gaels in this period, it's always either too early (6th-9th century, pre-Norse) or too late (12th century +) I'll see what I can do though. I should have some more detailed info on the military as a whole and on what equipment would be like, as well.
Lancer/Antagonist