It is quite a distance, you will need to take a taxi. Every taxi driver knows where the Sun Set Beach Bar is, so that will not be a problem.
One note of warning, traffic can be very heavy on the Island from the airport to the cruise line docks and there is a drawbridge on the road from the airport to the dock. So allow plenty of time to return to your cruise ship.
By the way, unless it has changed the Sun Set Beach Bar does not serve Carib Beer, so don't ask for it, or you'll get a dirty look. They do have Bud, Bud Light, Heineken etc. The bar takes US money, the whole island does.
Also, unless it has changed in the last few months the bar does not take any type of credit card, cash only!
Oh, don't forget to take your camera.
Have a great time.