.....must be the Trinity College Tour.
This is run by students and costs €10 INCLUDING entrance to The Book of Kells which costs €8.
so for €2, you get shown round the college with the various buildings pointed out and anecdotes told about them.
It beats just wandering about looking at the buildings.
There are quite a few tours during the day, it lasts 30 minutes and ends a few yards from the entrance to the Book of Kells.
Note: you don't get priority entrance with the tour.
The young guy (a second year student) who took our tour was great.
You can do the tour without the Book of Kells entrance fee but it costs €5.