Check out the Slane Castle website (link below).
Book your tickets online at Ticket Master (link below). General admission is 86.50 Euros, but their is a Gold Standard ticket that gets you closer to the front for 106 Euros or tiered seating tickets that costs between 116.50 to 131.50 Euros.
Plan how to get there. Slane Castle is 30 miles from Dublin and for detailed directions go to Google Map (link below). Alternatively, Dublin Bus are running a special service to the festival site for 20 Euros. For information go to the Dublin Bus link below in the resources section.
Book accommodation for the weekend. You can actually stay at the castle itself, but you need to book through D Hotels at the link below. Make sure you quote Slane Castle. Alternatively, a cheaper bed and breakfast, hostel, or a hotel can be found in Dublin at the tourist information link below.