Yes, IRREDENTISM is a Scrabble word!

Letter values in Scrabble US: I 1, R 1, R 1, E 1, D 2, E 1, N 1, T 1, I 1, S 1, M 3.

Is it a valid word?

IRREDENTISM is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the doctrine that irredenta should be controlled by the country to which they are ethnically or historically related

Not playable in Scrabble US: IRRIDENTISM.