Are there certain locations in Dublin that end up with long lines waiting for tickets? Can tickets be pre-purchased at any of the sites?
Just look at the websites for the places you want to visit, and see whether there is an option to register/purchase tickets online. For example, the Trinity College Library (to see the Book of Kells) offers advance tickets. Whenever they are available, and I can be reasonably sure of sticking to a given time and date, I get advance tickets online. Can't hurt, and it might help.
Advance tickets to Kilmainham Gaol are a must. I believe they become available 30 days ahead. They sell out quickly. Advance tickets are also available for the Guiness Warehouse and 14 Henrietta St. Guiness Warehouse lines can be long (think Disney Land).