Near Glenveagh National Park. Park at the car park on the south-eastern side (not marked on map), or at the Dunlewey Centre.
Surprisingly, this is nearly all rideable from the summit, with extreme caution required on the higher rock section. Unsurprisingly, it's a carry the whole way from the car park to the top. Not really a trail, just a big hill.
A bit of advice:
Stay on the main route on the way up. I.e. don't get drawn onto any of the paths that go round to the right. Stay straight and true, and head on over the ridge, as the other way is just a bit hairy when carrying a bike.
CAUTION 1: Do NOT attempt this if you are not fond of very steep, slow, tricky, technical riding, and have lots and lots of experience, because you will hate it and will probably get wrecked. This is not one for someone who has a go on a bike occasionally, and just fancies a fun day out.
CAUTION 2: Riding the ridge is a BAD idea. The ridge on Errigal is dangerous enough on foot - folks have gone over it and been killed!
CAUTION 3: Don't do this challenge on a windy day - seriously, it is a very bad idea - the wind catches the bike like you wouldn't believe.
OSI Map 1
CURRENT AVERAGE
3.0
OUT OF 5 BADGERS