Romanians are the decendants of Dacians and Romans.Dacia (as Romania was anciently called), was populated by Dacian tribes and Celtic tribes before 101 A.D.Around 106 A.D.the Romans started their massive collonization effort trying to exploit the gold mines of the romanian mountains.Most of the roman collonists were males, who were given lands after their military service.Most of the male Dacians were taken as slaves by the Romans and deported to various parts of the empire and forced to join the various legions there as soldiers, slaves, etc...
So the Roman province of Dacia, ended up roughly with a Roman male population, and a Dacian female population.You get the idea, as to what happened later, and that's how Romanians got born.
The Romans I don't think immigrated from Dacia, or from anywhere around there.Not in the past 3000 years or more anyway.
The Dacians were a people very closely related with the Celtics, who at the time lived in around that area (present day hungary, poland, slovakia, etc...)When the various migratory peoples came the celts moved westwards towards france & britain, and ireland, etc... to join other celtics there.
Because of Romania's geography (plenty of mountains, and rivers).The dacians, and the ir romanian decendents were not dislocated, and managed to survive on their native lands.