Twitter claims that the move is being made in order to better support users around the globe. This may seem like a good cover, however, there’s likely more to it.
New Jersey Spotlight covered the moving non-US data outside of the states will allow Twitter to keep foreign traffic out of the prying eyes of the NSA while remaining compliant with US laws. Out of sight out of mind seems to be the best policy when dealing with the United States government.
Another reason for moving international data to Dublin could be softer privacy laws in Ireland. This would allow Twitter to share more private information with advertisers to increase profit margins. This is a nice little trick to side-step US laws regarding user privacy.
Yet another explanation may come from the turbulent future of net neutrality in the US. At this point it’s still unclear whether ISPs will have the right to charge more for faster access to their networks. Being outside of the US will completely avoid this issue for non-US traffic.