Many people rent a bicycle when they get off the ferry boat to ride around the island. Which is nice especially if the weather is nice and you have enough time. It rained very hard in the morning of the day we visited, and we only had 3 hours to spend on the island, so we decided to take a mini van tour to visit the main points on the island. It was 10euro, which is the cost of 1 day bicycle rental.
We left Kilronan, the main port of the island, went to Dun Duchathair, Dun Aonghasa, the West point of island, Na Seacht dTeampaill (seven churches), by the Kilmurvy Beach, and back to the port.
The guide/driver gave us good information about each place, and was nice to cover all the points we did in a short time.
You’ll notice these as soon as you get off the ferry. It is a good option for people who are day tripping, since Inishmore is fairly large and the minibuses stop at the main sights. They can also be used as a taxi, though the prices are pretty random (they are negotiable). It is better to let the tour groups go by first, if you are using it as a taxi. Along the ride, the drivers give a (half-hearted) description of the sights that they pass by.
There are vans that take you to the main sights of Inishmore waiting in Kilronan. It costs around 5 IEP and you will see a lot of the island, get inside information (like: 'The oldest woman on Inishmore is 109 years old and that's her house over there') and you will be brought to Dun Aengus and to the 7 churches at least.