The book of Kells is not really a tourist trap, but it is kinda pricey to go see it (around 9e). I've heard the library that contains it is very nice, though. PS- if you do get in, you only get to see 2 pages...beautiful pages.
Fun Alternatives: There's a hotel across from St. Stephen's Green which contains a room where the Constitution of Ireland was signed (I forget the name, but will get it up asap).
In the hotel, they have one of the few original hand-copies of the Book of Kells on display unassumingly right next to the entrance. Plus they have live piano and a bar next to the lobby. ; )
I do not know if it is a usual practice, but we arrived 20 or so minutes before the Book of Kells exhibition closed and were let in for free. With that behind us, we were rather impressed. I agree with previous tips that the Long Room was more interesting than the book. I found 20 minutes to be plenty. I fear I would have felt very very disappointed had I paid to see that.
We went to visit the trinity a couple of days ago, I have to say I was impressed with the campus itself, so full of history. Couldn't help but thinking how lucky those students walking by us were to be able to study in such a historical place. The tour before the library in which you are explained about the Book of Kells and other historical ancient irish books is so interesting and the sight of the Book of Kells itself is amazing, with such a historical background and the explanations on how it was written it makes you think on how those people spent days and months working on it. It makes it a incredible work of art.
I was also deeply impressed by the library itself, the long room. We even spotted in the bookcases some ancient editions of the Civil Compilations by Justiniano (me and my friend have studied law, so we were impressed to see them) and a first edition of Dante's Divine Comedy. It is worth a visit.
Trinity College - Book of Kells. (see http://www.tcd.ie/Library/kells.htm)
This place really pulls the tourists in but there's actually not much to see. It's not that it's uninteresting or badly exhibited, just poor value for money and very crowded. If you're on a budget but love old books, skip this & go to Chester Beatty Library instead.