Oh oh oh!!!! I love Slieve League! Okay, so I went from Donegal to Galway, so there are busses between the two. Obviously. But the bus from Donegal to Carrick (or Kilcar, I was in between the two towns at an adorable little hostel called Derrylahan Hostel) I am pretty sure only leaves at 3 times during the day, twice early in the morning (Read: 8am or 9am or something) and once at night (9pm or something). I would suggest looking that up because my information is about 9 months old. Alas, you will probably not be able to make it there tommorow after you bus to Donegal, so spend the day in Donegal (it's nice!) and then go the next day (and stay in a hostel nearby, I reccomend Derrylahan, personally!)
My hostel was across Teelin Bay, which is a long, thin bay, and although Slieve League LOOKS closeby, it would have been a 6 hour walk around the bay. So I hitched/mostly got offered lifts by people driving down the street. From Carrick, it is a little closer, but trust me, just walking up the 'driveway' to the parking will tire you, and if you want to hike up One Man's Pass (you should do it, by the way) you'll need your energy. Hitching is fine in this area, along that road it is mostly tourists headed for the same thing anyway. :P
The cliffs aren't hard to find... its the mountain that looms over the entire area. You can't miss it. The bus turns around a corner and you're like "What is THAT?". It's the cliffs.
Personally I would suggest going by yourself, mainly because I have a sneaking suspect that any tours you get will just take you to the parking lot, which has a pretty lookout, and then go back... half the fun is walking along the cliffs (Not too close to the edge though). There are paths you can follow, and the views once you get up there are very rewarding. 
