Facebook already has 400 employees in Ireland, mostly handling the business side of things
Facebook is expanding its operations in Dublin, Ireland where its European headquarters are located, by adding 100 new positions to complement the 400 or so employees it has there already.Most of the positions are either in sales and user relations while some are data center employees, meaning Facebook is only expanding on the things it already does in Ireland.
Facebookers in Dublin mostly handle the business side of things, but the company also has a big data center there. Even huge data centers don't have that many employees though.
Dublin is already home to plenty of US tech companies, attracted by the favorable laws and taxes in the country, as well as the skilled and relatively cheap labor.
Facebook Dublin was established four years ago when it had just 30 employees. Facebook is hiring at a fast pace at its main headquarters in the US as well.