[Gil]

Well, the role of wars in defining dominant populations has been at times over emphasised and sometimes put down as irrelevant. I think the main issue is the one you refer in the end, about population density. For example, in the Irish case, if the norseman who occasionally raided their shores would have been organized and had attacked at the same time instead or irregular waves one could have had a scandinavian Ireland today because the local populations were never very high, specially because they kept fighting among themselves.
On other cases like Iberia it's the exact opposite. For example, when the Moors started conquering the southern provinces and advancing north, the local populations were not exterminated simply because the conquerers wanted peasants to work the fields and because theonly threat to their rule were the ruling class. With the visigoths disbanded or killed they couldn't care less about the indigenous, and so they settled more in the Al-Andalus than anywhere on the Peninsula. Of course there were marriages between indigenous and moors, but their degree was less than most people think. Also, when the Reconquista began the knights had no bias at slaughtering vast amounts of mixed people, just on the basis that they were "moorish".
I can tell you that in the case of Portugal, for example, by 1250 the Portuguese Reconquista came to an end as we had conquered all the regions down to the Algarve and our borders with Spain settled to almost how they are today. And what happened to the local populatons? Well, in the major burghs each town had a Mouraria, a ghetto neighbourhood where the moors or converts to Islam could live. They could neither trade nor inhabit outside that neighbourhood, unless they choosed to live in the countryside. What happened was that certain regions of Portugal, specially to the south (Alentejo and Algarve) there is an abundance of darker phenotypes than the rest of the country, probably because they are descendants of those outcast. It's not like I consider them to be less portuguese, only different.
My point is that it's impossible to make definitions applying to a whole country, simply because there is local variance, regional variance and continental variance (not to mention global variance).