How to plan a day out to the mourne mountains?

Hey, me and a few friends are planning to take a day out in Newcastle, Northern Ireland to visit the mournes, we will be travelling from Belfast. I was just wondering is it free to just walk onto the mournes as soon as we get down there? Also is there any free parking? And last but not least directions to getting there from belfast? I have a fair idea of how to actually get to Newcastle from Belfast but not the mournes. Thanks!