To whistle an Irish traditional tune all you need to know is how it sounds. You don’t need to know the tune’s name, what key it’s in, or even whether it’s a jig, a reel, or some other type of tune. All you have to do to is match the rhythm, phrasing and pitch changes – in that order of importance. There’s no need for notation and no need for an intellectual understanding of music. You do it all by ear, letting visceral intuition, not logic or theory, inform note selection.
Children learn how to speak a language fluently before they even know how to read it. As an adult learning to play music - essentially a second language - the same notion applies. You don’t learn a language by studying it, you learn it by speaking and listening – slowly grasping the peculiarities of communication without needing to know the grammar rules behind it. Being able to read doesn’t help you speak the language and it certainly won’t help you understand it when it's spoken to you.
It will be hard at first. You won’t know how to do it, it will be embarrassing; you’ll feel frustrated. But eventually you’ll be able to express yourself by making the finger movements necessary to create the sound you want to come out. Keep at it. Don’t worry about putting yourself out there, hitting a wrong note, or failing. Those thoughts and fears don’t enter your mind when whistling a tune so why let them dictate the way you play your instrument?