There is a walking literary tour which is interesting and very enjoyable. It combines info on many of the famous Dublin authors with a bit of a pub crawl and is great craic. I don't have the info on it but I'm sure you would find it on a Dublin tourism site.
You might also find the iwalks section of this site interesting
http://www.visitdublin.ie/. In fact that site seems quite good for lots of general info.
The sightseeing bus is a great tour and if taken at the start of your holiday, it help to give you your bearings of the city as well as allowing you to hop on/off anywhere that you would like to explore a bit more.
Of course you just have to have (at the very least) one pint of guinness.