As for the Book of Kells, it is truly a treasure, but keep in mind that (1) on any given day, only one page of the book is open for public inspection; (2) it is in a very dimly lit room, the better to preserve it; and (3) there are crowds of people waiting to see it, so you have to stand in line, and then they discourage you from spending more than a few seconds in front of the display case.

Since you have such a limited time in Dublin, I recommend that you skip Trinity Library and instead follow up some of the links on this page.

The images are excellent, larger than life, and you can study them at your leisure.

Here's a bit of info about Kilmainham Gaol. Also, the site called Celtic Camera®: The Ireland Portfolio has some nice pictures.