It's spread all over the place, GSR; there isn't any one main central online repository about most aspects of Irish railway history. If you check out th likes of the photo website flickr, you'll get some superb contributions. In some cases these are made by people who post here.
If you're anywhere close to Dublin, there is a lot of ("offline"? - as in not computer based!) material in the archives of the Irish Railway Record Society, which are open on Tuesday nights if you're a member. There is a good selection of photos and books available there too. The GSR was Ireland's largest company by a very long way, in pre-nationalisation days. It might be useful if you could give a few ideas about what particular aspect of the GSR you are interested in. If it's for a layout, then the types of locos and coaches used. You might like main line operation, like modelling the Galway, Cork or Waterford main lines. or you might prefer a sleepy rural terminus. If the latter, this could be of ex-DSER, ex-MGWR, ex-WLWR or ex-GSWR origins; all had their differing styles of architecture, but all were under the overall GS banner fro 1925-45. Because of this, there was also vast diversity in the range of locos and rolling stock used.
Then there was the narrow gauge - at its inception, the GSR inherited no less than seven separate 3ft gauge systems, four of which it retained to hand over to CIE in 1945. It narrowly missed inheriting the Blessington tram as well. The separate and self-contained Waterford & Tramore system, ad the "as-good-as" separate West Cork system were all part of the GSR.