phyllisbrookss has it exactly right. I too have been haunted by the memory of seeing Pennywhistle Blues in the 50s. I have also been haunted by its opening theme, which is running in my head right now. I am delighted to learn that it can be found on tape. It ought to be re-saved on DVD as well, so that it does not follow tape into obsolescence.
As has already been remarked, it must be a superior film if it lingers in the memory for 55 years. I have recalled other films this way, however, and have been disappointed when I finally saw them again. An example is O Cangaceiro, with its splendid opening scene as the bandits appear coming up from behind a ridge in silhouette. That scene turns out to be much shorter than I remembered it, and one hears far too little of "Mulher rendera" as a result. Other music in this film does not last as long as I recalled it, either.
So I hope that the music I remember from Pennywhistle Blues lives up to my recollections of it. From previous comments, it seems that it may.