Alek: In the cities I lived in (Bielefeld, Cologne & Frankfurt), the general rule was board by both doors until 20:30, from then on (presumably when it's nice and quiet) board by the driver and everyone has to show tickets. In practise the drivers don't really even look at you when you present them your ticket so you might as well show them a naked picture of Britney Spears... But still the rear door works to let people off.
For the germans it's not very logical, as people who don't buy tickets would travel during the day
But then again they probably realize that the direct & opportunity costs of a bus sitting at a stop for 5 minutes while people board via 1 door and have to either buy tickets or stick an existing ticket in a machine that always seems to go "beep beep beep" are too high and that people (given a certain standard of living) are generally honest and will buy a ticket. And as Kevin said there are quite a number of undercover "revenue protection agents" (I love that Euphemism) i.e. Ticket inspectors floating round during the day who will whack an instant 60EUR fine on you if you don't have a ticket.
Well I still haven't bought a car so I am left at the mercy of Public Transport (or my trusty bike).
ps. I am travelling to Kilkenny tonight, train will probably be chockkers. In Gerrrrmany (roll r), I would have reserved a seat over the internet, paid for and printed my ticket out. But that is obviously too much to ask from IE