It all depends on the time you will be visiting. While it is possible to buy your tickets on arrival, if you're travelling during the busier summer months, or plan to take the ferry back in the weekend or around a bank holiday, it would be advisable to prebook your tickets online. In most cases, tickets bought online will be cheaper than those bought on site as well. If you don't want to be stuck to one date, you could make a flexible booking for a little extra.
We've had a similar thread
started by Rosemary some time ago, which you might be interested in.
How's your itinerary looking for your 4-5 weeks on the island? If you're in need of some extra tips, you've come to the right place! :)
Mocht je willen, helpen we je ook graag verder in het Nederlands trouwens. :)