I think that migrating a web forms applicaiton to MVC is going to be very hard unless you have a clear seperation of concerns in your current applicaiton. If you have followed a design pattern like MVP then it might be easier, but if not then much of your business logic is likey going to have to be moved to controller classes and much of it re-written.
I would start by extracting your model, this should be fairly easy, then identifying your controllers and actions and seeing how much code you can re-use. At this point you should be able to discern whether or not you can migrate or if you'll be better off re-writing portions of your applicaiton.
Default URL patterns in ASP.NET MVC are http(s)://(appdomain)/(controller)/(action)/(par/ame/ters)
So your url above should fit into that pattern. You can change the pattern to account for other things (like namespace for example). Your URL pattern might be:
http://www.eireads.com/cars/used/ireland/mayo/citreon
where ireland, mayo and citreon are the in put parameters.