It is becoming more difficult to obtain Locomotive passes except on official business. However, Irish Rail are quite tolerant of platform photographers, unless you are obstructing, or do not have a valid ticket (ask if in doubt).
Heuston station is excellent for observing passenger workings; it is usually possible to gain access to platforms 2 and 5 unhindered. While platform 10, situated at Islandbridge Junction, is only a relatively recent construction, it is currently out-of-use. If you want to see stock in the yard, one possibility is to walk towards the car park at the north-western side of the Heuston complex. It is not really suitable for staying for a long period, since it is video-monitored and staff might think you are intending to damage or steal vehicles.
I wouldn't recommend the first station on the Arrow/Kildare line which is Cherry Orchard/CityWest since this is one of Dublin's troublespots and expensive camera equipment (or people loitering on platform ends) are very likely to attract attention. The next station, Clondalkin, is usually OK but I heartily recommend a trip to Hazlehatch as it is normally a safe place from which to observe. There is also a loop just west of the station and this can clearly be seen from the down platform, or by walking to the nearby road bridge. Trains pass through at up to 90mph so care is needed when standing on platforms. Kildare is also worth the effort, since most Arrow DMUs terminate here and there is a passenger loop on the up side. There is sometimes some civil engineering action here as their sidings are just off the down platform. The junction for Kilkenny and Waterford, Cherryville junction, cannot be seen from the station - however, there are good photographic opportunities available from the platforms.
It is possible to visit Dublin's freight depots at North Wall but vigilance is required, particularly if you are sporting camera equipment. You are quite likely to attract unwanted attention if you are not careful; having said that I have never been troubled here (although my pushbike is always ready for a quick getaway!). It is not so easy to find good vantage points for spotting or photography here. I would recommend the West Road overbridge though [map section V-43]. If travelling from the city centre, follow Amiens Street north which then becomes North Strand. Turn right after crossing the canal and railway onto Ossory Road - you should see the old signal cabin and the Northern lines ahead. Follow the road to its end where it forms a right-hand junction with West Road. A short walk to the overbridge will take you to one of the best vantage points for freight and passenger workings in the area. Be warned - there are not many freight workings to/from Dublin and they tend to leave Dublin in the evening (about 1830 - 2200) returning in the early hours. From this point you can also see action on the Northern lines and the Loop line [which also includes Maynooth/Sligo traffic as well as light engine movement from Inchicore shed].