Comparative form of tired: more tired.

Is Tireder grammatically correct?

"Tireder" isn't formally correct in written English, but you can get away with it in spoken English (as Joanvillafane so kindly pointed out, with her LC quote "behinder"). Consider it a humerous way of saying it, but it is acceptable in spoken English.

Is Tireder a Scrabble word?

Yes, tireder is in the scrabble dictionary.