You need to find the junction of R251 and N56, and head north. From most approaches you'll be taking R251 west; take this past Errigal about 3 km, then make a sharp right onto N56. You really need the OSI #1 map, but I'll try to give directions.
After turning north from the junction, take the second right (5km north of the junction). You'll now be on some narrow roads. Take this road 1km until it dead-ends at a school, then turn right. You'll undergo windy bends and pass turn-offs that could either be driveways or roads (this is why you need a map). After about 2km, turn left, you'll cross a lake on your right, then take the third right. Tracks lead south towards Errigal, and you'll want to park at one of the lay-bys between the first and second tracks. Pull off the road as much as possible, then park. You don't want to block tractors - they blow through all the time. The relationship between climbers and farmers is still good in this area. Let's keep it that way.
Follow the track from either turnoff as far as possible. It will end in a hairpin turn, linking both turnoffs. From there, it's a 1.5 km slog upwards through boggy terrain. The scree slopes will then begin. The loose rock is relatively stable, but you'll need to use hands in some places when you hit a suddenly unstable part. Keep to the north ridge until the scree fades and you'll hit good solid quartzite rock. From here you can pick your favorite way up, making it a class 4 staying completely on the ridge, or traversing right as you go up, lessening the difficulty. There is also some class 4+ later on if you traverse right then head up.
Traversing right, you'll cut across some gullies, and come across a peculiar rock arch - the only kind I've seen in Ireland. When reaching this, you can turn left and head up for some vertical scrambling, or go through the arch, cut right and gradually gain the ridge. From there it's a simple walk up the remaining ten meters or so. Head down using the Tourist Route, and try to find a ride back to your car. ;-). I found another hillwalker at the top of Errigal willing to do this. Otherwise, you can walk back over the boglands or walk some pseudo-roads 5km back to your car.
Waterproof boots and a layer of waterproof clothing. An OSI #1 map is very useful for driving, and indispensable for climbing when foggy.