If you want to go to Ireland
There is no way around to go to the Ferrystation and buy a ticket. I asked at the parking lot, nobody was willing to take me into the car, as the ticket is per person. Also to hide somewhere will not work out.
If you come from the ferry from Ireland
Most likely you are a foot passenger crossing from Dublin to Holyhead. When you are in the ferry You can ask people for a ride. But you have plenty of time to prepare a neat sign! Don't hesitate to write London, if you want to go there, although it is far. But the best thing is, people will watch you writing the sign, and for sure they will ask if they can help.
If not, get out of the ferry as soon as possible! Yes, run! Get out of the ferryport, towards the exit, there will be a roundabout. You can try to get a lift here, but a better spot is, when you keep on running for 50 metres more. There will be a nice spot, where all the cars passing by can see you easily. There is also a parking 15 metres ahead - perfect.
The reason why you should run and hurry is simple explained: Cars from the ferry are more likely to go far to major cities. If all cars are out from the ferry, you may have to walk to the motorway (which is close) and hope for better luck. So hurry up, every car counts!
Bernhard waitet 15 minutes and got a lift straight to London, a lucky one.