From Apple’s email this afternoon:

As of September 25, 2016, iTunes S.à r.l. will complete its merger with Apple Distribution International in Cork, Ireland and will relocate its business from Luxembourg to Cork in early 2017.

Apple has been operating in Ireland since 1980 and now employs nearly 6,000 people. As we continue to expand our operations in Cork, we are moving our iTunes business there and will support content stores for more than 100 countries from our campus at Hollyhill.

This move was originally announced back in June. From this 2012 article in the New York Times:

Luxembourg has just half a million residents. But when customers across Europe, Africa or the Middle East — and potentially elsewhere — download a song, television show or app, the sale is recorded in this small country, according to current and former executives. In 2011, iTunes S.à r.l.’s revenue exceeded $1 billion, according to an Apple executive, representing roughly 20 percent of iTunes’s worldwide sales.

Obviously, that information is dated, but it gives a sense of the importance Luxembourg operations have to Apple. Not clear how this will impact Apple’s current tax negotiations with the European Commission.