I've been told by a friend who goes to Castlewellan often that there is only a part-time ranger there, but that you can book through Tollymore which has a full-time ranger, or he might be able to tell you how to contact the Castlewellan Ranger.
https://www.nidirect.gov.uk/contacts/contacts-az/forest ...

I hope to go as well and I too am heading down from Larne.