The Isle of Man birch was, if memory serves me right, composed of a number ( 3 or 4 ? ) rods of hazel bound together, and was 40 to 48 inches long, depending on whether it was for adults ( over 18 ) or juveniles.
I recall reading an interview with one of the policemen who had the job of making up the birches when required who admitted that he took care to adjust the expected effect to match the severity of the crime ! I also remember talking to someone who has some experience in the infliction of corporal punisment in various ways who said, in a tone of awe, ” The Isle of Man birch – now that REALLY hurt” – the way he said it made me think !