7.5 finished is a very unusual size for a finished block. When we say finished that is the size of the block once it is sewn into the quilt. 7.5 is a more common UNfinished size (which is what the block would measure once completed but before it is sewn into the quilt)
So if 7.5 is correct that means before your block is sewn into the quilt it will measure 8" and Irish NY sizes are correct. But if you meant to say 7.5 unfinished size then your 4P squares need to measure 4" each.