Hello and welcome to the forum and good luck with the Kerry Way. I don't want to sound like a wet blanket, but doing the Kerry Way (203 km) at very short notice is an ambitious project, esepcially in the height of the tourist season.
Being spontaneous is lovely, but August is a very busy month, and if you are relying on B&Bs, beware: many have only a few bedrooms, and they may have been booked out for months. Even if you find space, you may find only the most expensive or out-of-the-way B&Bs have any rooms. Be sure to phone first or you may walk extra miles only to be disappointed.
On arrival in Killarney (if not before, which is what is advised) go to the Tourist Information Centre
If your budget is limited, look at hostels, both An Oige
and Independent Holiday Hostels
- just click the link to visit the website. If the hostels are full on the dates you need, it's probably going to be difficult to find a bed elsewhere.
Land in Ireland is privately owned, but there are campsites on or near the way at Killarney, Glenbeigh, Cahirciveen, Caherdaniel, Sneem and Kenmare. Visit [url-http://www.camping-ireland.ie]this website[/url] for details. Also some hostels will allow camping outside, but you have to ask first. For baggage transfer, a big help when camping, try Irish Ways
or Trek-Inn holidays
I hope that helps, and do let us know how you get on. Good luck!