Irish Rail have nothing to do with construction (not that they would of managed it on time either!), its the NTA or TII who are in charge of it and they had 4 years.
One story I heard (maybe not true, but totally believable) was that work on the connecting footbridge was delayed because of a dispute between Irish Rail and Transport Infrastructure Ireland about who should pay what share of the cost.
Pathetic, but totally credible.
Not sure about the dispute but I agree IE shouldn't have to fund the project. After all it's part of Luas project. For example it was Luas/TII/NTA who fund access to Connolly station and maintain it today.