It is too tight. The journey is under 15 minutes from stop to stop but that isn't accounting for getting off one train (which on the plus side may be up to 5 minutes early getting into Heuston); time from the train to the Luas platform (1 minute minimum for platforms 2-5, 2-3 minutes minimum for platforms 1, 6, 7 & 8); time for the next Luas (up to about 7 minutes wait on a Saturday); the Luas going to The Point instead of Connolly (get off at Bus Aras and walk across instead of waiting) and the time to get to your next train. Irish Rail seem to recommend a minimum of an hour, truthfully 30 minutes is doable.