I've met people who don't go in to Dun Aengus because they don't want to pay 3 euros for admission. Having seen it with my own eyes I can't image how some some could over look this beautiful spot over such a small amount. You can jump the fence if your that broke.
The fort has three stone walls with the inner most wall being the thickest and highest. Once inside I found my self imagining the goings on of the people who built it 4000 years ago. The fort is right not the edge of one of the islands highest cliffs. It's a great spot for pictures if your seeking one of those sterotipical ireland cliff photos. If you have to choose between the cliffs of more or coming here, inish more has more to offer.
If your biking here, try sticking to the south side of the island on your way up here. Take the normal route north west of Kilronen, then when you reach the bottom of the hill where the light house sits, climb the hill and take the back roads. You'll be glad you made the extra effort with fun down hill trails most of the way to Dun Aengus looming in the distance.