Some people instinctively pick up their first whistle and put their right hand at the top, although not wrong it may have some issues down the line.
First off, it wont matter if you put your left hand or right hand at the top, as long as you can play the notes, your tunes will sound just as good as anybody else.
But....lets say down the line you want to progress onto the flute or the pipes which is a natural progression in Irish music, then you will have a problem! In order to get the notes out of these instruments you must use your left hand, or your arms will be in a tangle.
So beginners, when your starting to play, use your left hand for the top holes on the whistle and save your self the hassle of getting in a tangle later on!