[doctorwho]

It's going to come down to what kind of business the Irish company carries out on behalf of the parent company or whether the Irish company is the parent. There are so many ways the relationship could be set up and figuring it all out could take a loooong time. First they have to work that out before they can determine whether they are responsible for following EU law and what laws they are obliged to follow. The Irish company may not have anything to do with data which would pretty much let them off the hook I'm guessing.

[rmc]

Well anyone who signs up to Facebook who's not in USA or Canada is technically signing an agreement with Facebook Ireland Ltd. So, the Irish company is responsible for the data of hundreds of millions of Facebook users.

(cf. section 19 of https://www.facebook.com/legal/terms )