They aren't true prawns — the langoustine is a small lobster, which is why the same animal is also sold as Norway lobster and scampi.
These langoustines are simply boiled or grilled and served with brown bread and butter or a squeeze of lemon, letting the sweet, delicate meat speak for itself. Dublin's fish stalls and seafood shacks sell them by the bag, shell-on, for cracking and eating by hand.