What's the significance of this? Twitter Inc is governed by US law, it is obliged to comply with NSA-driven court requests for data. Data stored in Ireland is not subject to the same obligation. Twitter is not alone in using Dublin as a base for non-US operations; Facebook is another company that has adopted the same tactic. The move could also have implications for how advertising is handled in the future.
Ireland is widely recognized as having the most relaxed privacy laws in Europe, and this is something which is important for any company looking to monetize user data through advertising. While data that is processed in Europe may be beyond the immediate reach of the NSA, the policy change could also be seen as a way of trying to sidestep future legislation which may make it more difficult for US-based companies to share data about European users with advertisers.