I booked our May 21st trip with Catherine the other day, so I don't have to worry about remembering when we get to Killarney. They don't charge your CC card until you arrive.
It is real easy to book with Dero's Tours. They have a small office right in Killarney town or your B&B or tourist office can book it for you. A day ahead sounds fine.
It's 46 days for us! Please make sure that the great weather you will be having lasts for us! Since last May was rainy, perhaps the good weather gods will be with us this year. Ah well, rain, sunshine, snow, sleet will not spoil any Ireland journey. Just being there is reward enough.