There are a few good spots to use,the long stay car park parallel to the main runway is good,and you can climb up the bank to get a better view-not sure how much it is to park in there,but it`s secure.
There are other area`s around the airport,but be careful not to block any crash gates or access.
The USA flights usually come in early,mid morning is Ryanair Fest.Beware of the passing lorries and buses,that can ruin shots.
Doubtless if you search for EIDW spotters you will find some useful info.