You could try the Caravan club CL. Not sure if they take non members. I know some of them do especially out of season.
http://www.cushervalleycl.co.uk/
Cusher Valley Certified Location (CL), Markethill
Cusher Valley is a small 5 pitch caravan site for Caravan Club members only. As part of the CL network, the site can accommodate 5 caravans, motorhomes or trailor tents and is open all year round.
It is situated off the main Armagh to Newry Road (A28) and between the small country towns of Markethill and Tandragee (B3) in County Armagh. It is ideally suited for a cross border stop off between the ports of Dublin and Larne/Belfast.
It is very close to the beautiful Gosford Forest Park.
Each pitch has hardstanding, electricity (16amp/8amp) and a water tap and waste water outlet. There is a shower/toilet and toilet on site.
Cost per pitch is from £14.00 per night. (including electric hook up).