Ireland isn't the tax haven, although it has a very low corporate tax rate of 12.5 %. They are in Ireland to channel cash to tax havens, such as Bermuda and the Cayman Islands, using the 'double Irish' and 'Dutch sandwich' tax avoidance techniques
Ireland also has a lot of high tech companies. If you watch the video you'll see they have several floors of staff, not just a brass plate. It's possible Facebook et al. are in Ireland for more than the tax breaks.