I tend to agree that if you can work up to 75-85 miles, you can probably do this ride. And if you end up not finishing it, it will probably still be one of the longest rides you've ever done, and good experience to help you prepare for the next thing you sign up for.

My friend hasn't actually done the Ring of Kerry, but she has heard that there are no major climbs and she said she'd be surprised if there was >2000m of climbing overall. She's also heard that people who live along the route are out offering food to the riders. I've done a few rides where people offered food, drinks and moral support along the road, and it's always a great pick-me-up.