Here's a piece that ran on NPR yesterday. The issue has more to do with US tax code loopholes, than with Ireland per se. Ireland has long had a low corporate tax rate as a tool for attracting FDI, and probably gets into more trouble with other members of the EU over it. If it was illegal, I assume it would have been outlawed. US likes the freedom to set it's own tax rates ........
It might get worse before it gets better. A wrinkle I've seen theorized is that if the UK pulls the trigger on Brexit, Ireland will be the only English speaking country in the EU and would be likely to attract increased FDI, and by extension more inversions.
Pfizer Would Cut Its Corporate Tax Bill If It Merges With Allergan : NPR