Keane for me by a distance. Followed by Paul McGrath. Anyone who suggests Irwin obviously never watched him play for Ireland. He was solid but never spectacular and wouldn't drastically improve our team as a whole in the same way a Keane or McGrath would. He also played most of his games for Ireland at right back and not left back as many on here seem to be suggesting.
As for Brady while undoubtedly one of the most talented players we ever had he was not a patch on Keane at the top of his game in an Ireland jersey. Keane was one of the few Ireland players that could almost singlehandedly win a game by himself.