I think you only touch on and off on Luas and Dart. When you touch on at origin it deducts a set amount, greater than the actual fare. When you touch off at destination it refunds part of it. If you register your card you can see all these transactions in the history of the card (leapcard.ie).
In Dublin Bus you only touch once (when you get into the bus). You can always go to the driver if you don't know how many stages you're going to travel. If you want to know in advance, you can check the fare calculators above or the timetables on the Dublin Bus website, e.g. at the bottom of [broken link removed].