Ireland’s favorable tax environment has long been a boon for Big Law. Thanks to the country’s rock-bottom 12.5 percent corporation tax, U.S. companies have invested more in Ireland over the past two decades than in Brazil, China, India and Russia combined, and more than 700 American businesses now have operations in the country, including Facebook Inc., Google Inc. and Johnson & Johnson.

Such investments are core drivers of business for Ireland’s big five law firms—A&L Goodbody, Arthur Cox, Matheson, McCann Fitzgerald and William Fry—who often team up with U.S. firms on big-ticket deals.