Maybe I can be abit of a help here. I was down in Dublin on Thursday doing a bit of spotting for the day on a day trip from Belfast. Unfortunately I am flying out of Dublin myself this Tuesday thus can not meet up. The location opposite the terminal and the start of runway 28 is good from about 9.30 onwards for most of the day, although before 9.30am, aircraft arriving can be caught taxing to the terminal in the opposite direction after departing the runway.
It takes roughly 30 minutes from the terminal to walk to the first spotting location. Just walk past the new terminal, and through the temporary walkways set up until you exit the airport, and then take the first right after passing 3 medium sized circular storage tanks. Follow this road for 10 minutes then take the first right. This road will be parallel with the runway.
The first spotting location is marked by a small roadside carpark, and a second position is a few minutes walk further along the road. These two positions are good for arrivals on runway 28.
If runway 10 is in use, and if you could be bothered, you could walk to runway 10 which is another mile down the road. If you couldn't be bother, then you can still catch the departures in the morning lighting, but be aware of heat haze. If it's sunny, heat haze can almost ruin any shots no matter how close the aircraft is.
If you need any more help, maps, or sample pics, let me know and I'll see what I can do! Enjoy your trip, and look out for an Aer Lingus A320, EI628 to Helsinki, departing at 9.40am