I'd be inclined to think that, if the Scotification of Caledonia by the newcomers from Ireland had been a particularly lively affair, that would have left its mark in the way of stories and songs and so forth. They weren't backward about boasting about mayhem. That would go towards suggesting that it might have been a relatively peaceful affair, as those things go.

As has been pointed out, linguistic and cultural changes by no means necessarily imply that the people living there previously have been wiped out.

I imagine the DNA crew will come up with answers to these questions based on a little more than speculation. (Even if these aren't maybe as infallible as they are made out to be.)