UKC

I'm looking into a group trip to the Mournes and would like to get any clues about where to stay for perhaps 20 people, some of whom might have vans. The guide doesn't have much info.

Could Meelmore Lodge perhaps be up your street? https://www.meelmorelodge.co.uk/ Group booking the entire hostel might even work for you. 

Not sure if Slievenaman Bothy is open at present but it could be an option if so: https://www.scoutsni.org/slievenaman-bothy/

The Hutt Hostel in Newcastle would put you next to pubs, cafes, beach (and the arcades!).

There is also a caravan-heavy, but pleasant, campsite within Tollymore Forest.

Meelmore Lodge with bunkrooms and campsite would be my first suggestion.
https://www.meelmorelodge.co.uk/

Hutt Hostel in Newcastle
https://hutthostel.com/

Thanks folks, much appreciated.

In case it helps, here's a couple of other suggestions. I used to take a university group out to The Mourne Lodge at Attical, it's set up a bit like a youth hostel I guess, and very convenient for hiking directly up to Eagle Mountain:

https://themournelodge.com/

Super helpful staff, very flexible and attentive, accommodating to dietary requirements, plenty of parking, it was reasonably priced and we'd book the place to have it to ourselves. The bar at the GGA grounds was a very sociable place after a day out too. It's a drive to get to any crags, Pigeon's the nearest (one of only 2 roadside crags in the Mournes), and the Carrick Little carpark (for getting to many other crags) is only a slightly longer drive.

Queen's University Belfast has its own hut in the Mournes, and it's available to be hired out by community groups / youth groups / school groups / scouts etc. It's pretty basic – you take your own sleeping bags & pillows, firewood and fuel. Camping indoors rather than a youth hostel. It's only 2 miles from the Carrick Little carpark.

https://www.queenssport.com/MourneCottage/

Thanks Joe, that's very useful. I contacted the Lodge and if we wanted to book the whole place it could get expensive depending on the take up for the meet, but it looks great and the staff sound super friendly and helpful. 

The Mourne Cottage looks like a good compromise.

No worries, hope it all comes together!

Depending on what time of year you're going, and what the group's focus is, it might be worth giving the Tollymore Outdoor Centre a bell and seeing whether there's scope to use their facilities? They run residential courses and they're set up with all the facilities and catering for groups. If you're rained off the rock you could certainly head there to use their indoor wall / bouldering cave. It's just a thought – I figure Meelmore Lodge probably fits the bill the bill better for getting out and about, and Tollymore might not want to provide an accomodation-only option, but I figure it's worthwhile for you to know what's nearby.

Fwiw, Castlewellan and Rostrevor both have very good MTB trails. Castlewellan has bike hire (and canoe hire too irrc). Rostrevor has the Kilbroney Christian Conference centre which is pretty close to the Rostrevor/Kilbroney Park trailhead, and it's available to "youth groups, uniformed groups, school groups, church/parish weekends, outdoor activity groups, adult retreats and conferences." You can do self-catering using their facilities if you've a group of at least 20. Not sure whether it'd fit your budget, but might be worth a look:

http://www.kilbroneycentre.com/adult/

It's good for access to the South West chunk of the Mournes, only 15mins drive to the parking spot for Hen if you're looking for some accessible slabs, and Rostrevor has quite an intense concentration of pubs for such a small town...