I had it all figured out ( so I thought), I went to Carpark info Dublin Airport : http://www.dublinairport.com/to-and-from/car-parking/long-term.html and studied all the rates.
As I was going for a week, the long term car park was what I needed.
If I booked ahead, It would cost me 7.5 Euro (normally 9.50 Euro)a day in the red cark park, and 6.50 Euro(normally 7.50 Euro)a day in the blue cark park.
I paid 7 euro per day. If I had not booked online the charge would have been 8.50 Euro, so I did save and I only had to remember the rownumber, not the colour of the carkpark:)
Happy travels( and parking)!