Certainly the game plan played a part.
When I watched it back, Brennan was involved in almost every attack that came from the backs. He was always available, and mostly used, as he's a good distributor. Gooch wasn't marking him, so when any moves broke down, Gooch was either free or Ger had just sprinted to get to him, so was pretty easily beaten 1-on-1.
O'Sullivan played completely differently in the 2nd half. Stuck to Gooch like glue and didnt look to get involved when Dublin had the ball.