In response to Ahmed's concerns I'd like to point out that nationalism did exist prior to 19th Century Europe. The names and forms that we associate with "nationalism" date from that time but ideas of cultural unity can be found in places such as the English and French colonies in North America in the 17th and 18th centuries.
Les canadiennes had national stirrings prior to the conquest. Power struggles between local elites and European Frenchmen sent over by the king were constant from the mid17th Century onwards. These conflicts became so bad that by the time the British invaded during the 7-Years War the French and Canadienne leadership couldn't organize for war, albiet they had a few early victories. Simply while the French claimed dominion and control over their Canadian colony the Canadienne likewise struggled, in a rather orderly fashion, for their relative autonomy (which they got, in a fashion, under British rule).
Basically if we take an isolated community that has developed certain unique characteristics that makes it fundamentally different than its neighbours, overlords, or some form of invader they become incredably self-aware fast. The nationalist leader would not come from no-where, it would involve some kind of society-crashing crisis, or a sudden realization of difference, that would shake up society in such a way that it recreates itself as something completely different than the other. ...
So.. maybe an Irish civil war and a Scottish invasion (or Breton, or whatever). The first will weaken the Irish state to the point that the invaders discover a claim on the Island and decide to take while the getting's good only to face a quick victor that manages to reunite the State behind his (or her) charismatic leadership as they realize their common uniqueness (not even consiously) in time to repulse the invaders...
Or something like that