Rain Gear, Killarney
Okay, this sounds very stupid, but mind the often unpredictable weather when you're out hiking of biking.
You might think little can happen, especially since the place teems with people in summer. However, lately two people got lost in the Kerry mountains as they were walking in dense fog. It took more than a day for the rescue workers to locate them so bring a good map and if you see fog or a rainstorm heading your way: take cover!