Scenery is the main attraction in the Southwest, and unless you're a biker or hiker, the best option for taking it in is to rent a car.
Once behind the wheel, plan to adopt the local pace—slow. Covering about 100 km (60 mi) a day is ideal, with many stops along the way.
Speed is dictated to some degree by the roads: most are small, with one lane in each direction and plenty of bends and hills.
Without a car, your best bet is to base yourself in Cork or Killarney, both accessible by train from Dublin, and take organized day trips or use the local buses (where available) from there.