I would recommend the blue tour as it takes the most in not just the city but the outer areas to like the cemetery and Croke Park. It is best to choose a bus with a live guide if you speak English as you will get a better idea of the history of the city and its buildings whilst sampling some good Irish craic. Worth paying €3 euros more for the two day pass so you can then visit the places you found of interest on the 2 hour blue line tour.

We got the 24 hour pass and thought this was a really great way to see the city when you only have a short amount of time. I think the stop can be more clearly marked but overall it was easy to navigate. We enjoyed the guides and really enjoyed the many brief rides we had from stop to stop.

Great way to cover lots of places in short time. There are various Dublin hop on off tours on offer. We bought 3 day freedom ticket (33 euro) that can be used on hop on hop off bus, any Dublin bus and Airport Link bus. It was a great way to explore the city as i only had 3 days and much of it was gone for business work.
I was on recorded commentary bus. There are two loops and walking tours which I did not have time to go on unfortunately.
Busses are frequent and last bus was 7pm (August) which gave me chance to explore city after work.