I didn't say people, Jack, I said cultures. Many of the indigenes acculturated to the dominant invaders.

After the German/Danish/Viking invasions: -

1. The old language disappeared
2. Stone buildings ceased to be built
3. What remained of centralised or regional government after the ROmans left also disappeared
4. Roadbuilding stopped
5. Paganism largely replaced Christianity (until the Irish brought it back)
6. Literacy largely disappeared (until the Irish brought it back).