WORTH IT. One of the most evocative travel experiences ever.
Went on the Celtic Victor out of Portmagee in August 2006, small open boat with rain/spray slickers provided (needed!). Seas choppy but fine; previous day's trips had been canceled because of high seas - you take your chances & hope. 50-minute trip.
Yup, it's 600 ancient zigzag stone-slab steps, vertiginous, no handrails. But sit on a slab & take in the phenomenal views & think about the monks while your heart/inner ear/lungs/legs collect themselves. Volunteer guide was up top to give info/answer questions, but unobtrusive. Easy to roam around/find a quiet spot & marvel. Incredible views & history.
Stayed in Portmagee - good room at The Moorings, gd Skellig Mist bakery, excellent fish&chips & gd roast pork at The Bridge, gd Skellig Experience Center, & the waterfront WC won Ireland's Top Toilet Award Runner-Up, there's a plaque. Maybe they've gotten 1st place by now.
Absolutely don't-miss-it worth it.