At DUB, like most airports now airside views are ok.
If you can get access to the Aer Lingus Gold Circle or Business class lounge. In Pier C there are good views through glass. At Pier A you can be very close to "Gulfstream Row", Ryanair use too, again through glass but with the possibility of snatching shots when the boarding gate doors open, which is fairly frequently.
Groundside views are less convenient, try restaurant area, also walk down to FLS at roundabout on exit road. There was an Ansett 767 there on Sun.
There are good spots near the threshold of the usual active runway, you can take a city public bus torwards Dublin (not the express or blue coach), pass ALSAA the EI staff club,
go on to a junction c300 yrds with a road on your right and a cemetary entrance on your left. Get off there.
Follow the road on the right, use extreme care as traffic is fast and heedless and there is no path, on to where some cars are parked and there are some mounds of mud to stand on , this should lift your view over a chain link fence at the point of touchdown.
Much further along this road is the Irish Helicopters helibase (hard to see into) and much, much , much further along are the light aircraft you can see on the other side of the airfield. You can make a complete circuit, which gives you a view of the front of FLS, but it is not a practical walk unless you do marathons.