널 알고 싶어요.- I want to know U.
널 알고 싶다 말이야- this sentence translates as:
-I mean,(that) I want to know you! or
-I'm tellin' ya that I want to know you!
You were right about the fact that it come from 말하다 in some way,
"말" means - words, talk.(말하다 ,말입니다,말야 =말이야, etc)
-you-know, you-see, that is,what I'm saying is ..., I mean, I’m tellin’ you, etc.
the dictionary form of 말이야 is 말이다.
I wish you progress in learning Korean!
Thank you! ^o^