The Scots/Irish/Vikings/Saxons/Northumbrians/Welsh and Picts are all factions in Viking Invasion. They do not need 'unlocking' as such, as they are not in the original MTW.

However, it is possible (I believe) to unlock certain factions which appear later on, The Golden Horde, Burgundians and Swiss. There could be some more but I rarely play the game that long!