So the general idea of an ethnic group isn't exactly scientific and can be defined along subjective lines such as "shared history" though they are often commonly agreed upon. Has there ever been an instance where two groups of people who considered themselves entirely separate entities with different cultures and histories became merged over time and began to identify as one ethic group?
As a completely made up example:
Two tribes in Ireland in 500 AD consider themselves to have nothing in common other than being on the same island are now both lumped together as "Gaelic" or "Irish."
hi! Springing off your hypothetical Ireland example, there have been several posts in here asking about "English" identity - check these out
First a little background, with layers of Beaker Culture people, Celts, then Romans
... add to that the Angles, Saxons, & Jutes
... stir in Norsemen
... and Normans