Is this mountain biking?

I'd say that was adequate to be called mountain biking, those rock gardens were probably rougher than they looked, and I know on my rigid I'd have been going at best 1/3rd of the speed he was.

Looks like they hardcored a landrover track and put in lots of mtb features. You could drive a car down that, widest trail I've ever seen. However it does look a blast if speed is your thing and its difficult to take the wrong line, a proper adrenline buzz and looks doable on our retro kit if somewhat slower

Don't 29" wheels and full sus make all trails like that one?

You are kidding, right? it's probably not as smooth as it looks on that video (the scenery was zipping by at a fair old rate ) and a full rigid bike would be left in the dust to be honest.

I went to Rostrevor last year, it's in the Mourne mountains, everywhere is called Slieve this & that (ie Slieve Martin), we had a good day of biking, including one descent where the discs and pads were quite literally smoking by the end!
I did see a couple of vans doing uplifts though, but I think that track must be fairly new, i didn't see it last time I was there.

You tube flattens trails out so much - try and find a steep trail you know on you tube and you'll see what I mean!

Repack was pretty wide.

http://www.youtube.com/watch?v=1_58GvTXxcU