It may sound an obvious thing to say but it is not how fast, but rather how slow, you go that determines journey times. In both our countries railway operators have settled for the semi-fast train, and have forgotten about the non-stop train. To get the best journey time between Dublin and Cork, or anywhere else for that matter, elimination of speed restrictions is the answer. Without allowing for acceleration and deceleration, 100 mins would be the minimum time achievable, say 2hrs in reality.
Stephen