Killarney National Park is a major visitor destination in County Kerry. The 26,000-acre park is known for its combination of mountains, lakes, woods and waterfalls under ever changing skies. Much of the park is protected land, but there are some wonderful, scenic trails. For runners, they range from easier multi-use paths to more rugged trail running. This page has a list of the trails, with good descriptions and information on distance & degree of difficulty. Some recommendations:
Knockreer Circular Walk. An easy 5k route starting in Killarney that include Killarney House and Gardens, and a section along the bank of the River Deenagh.
Ross Island Trails. Several easy trails exploring the grounds of Ross Castle, and the scenic Ross Island, out to Library Point. Can combine with Knockreer route above.
Muckross House Lakeside Walk. A good way to explore Muckross House grounds and adjacent area. 5k, easy.