I would recommend doing anything you can to get on a boat out there. I think I phoned about three different boats before getting two spots on a boat that was displayed as full online, but that was 11 years ago so things may have changed since then.
I think if you are traveling solo finding a spare spot could be possible, or even going on a reserve list.
It is a brilliant site, really unique so I would certainly recommend a visit.
It is perhaps wise to build in some contingency as the weather can stop trips. Kerry is a lovely place to explore, so if worst comes to worst then exploring the charms of the county are a great compromise if you are waiting for the weather to improve, there is even a t-list site on the ring of Kerry in Stague Fort.
I'm not sure what your logistics would be, but staying in or close to Portmagie is good for the night before sailing. Further afield I personally didn't like the town of Killarney that much it felt very touristy, Kenmare was much nicer, but your logistics may not work with that.