Brian Hayes, Irish MEP (member of the European Parliament) for the Fine Gael party put it bluntly. “That is the absolute red line issue,” Hayes told the Irish Independent. “Any attempt made to cajole us [on corporation tax], as far as I’m concerned, we’re out the door.”The door Hayes referred to was the one leading away from the European Union. The subject of leaving the EU has come up following the European Commission’s recent ruling that Ireland gave special tax status to Apple Inc. (NASDAQ:AAPLB).Related: WHAT TO DO IF INTEREST RATES GO NEGATIVEOn Ireland Leaving The EUClearly Ireland believes continued interference with its tax laws by the European Commission (EC) will cause irreparable harm to the country’s ability to attract and retain foreign direct investment (FDI).On the other hand, leaving the EU to gain total independence has its problems as well. For one, as Britain is about to find out, not being part of the EU could lead to complications regarding trade tariffs when doing business with former fellow EU countries.Other issues include free movement of workers, immigration policy and the impact leaving the EU could have on relations with Northern Ireland.Attitude of the Irish people on remaining in the EU has seemed mixed at best. One European Union expert said the impact of the EC’s decision regarding Apple would likely be negligible when it comes to staying in or leaving the EU.The Consequences Of Staying In The EUAssuming the Republic of Ireland and the European Commission can come to amicable terms regarding the independence of Irish tax laws – while maintaining fair and equitable treatment of all EU member nations – what might happen moving forward?Since Northern Ireland voted to remain in the EU, a firm decision on the part of the Republic of Ireland to remain, could rekindle talk of a unified Ireland. Such talk actually came up in July following the Brexit vote.Related: 3 MORE COMPANIES BESIDES APPLE THE EU HAS IN ITS CROSSHAIRSU.S. Companies In IrelandCurrently 700 U.S. companies have a presence in the Republic of Ireland. Sweetheart tax deals or no, Ireland’s 12.5% corporate tax rate versus the 35% rate charged in the U.S. make doing business in Ireland attractive.In some ways it would be better for those companies if the Republic of Ireland (or a unified Ireland) remained in the EU. On the other hand, not having the oversight of the EC might make negotiating tax deals easier in the event Ireland left.In addition to Apple, some of the big names with a presence in Ireland include Intel Corp. (NASDAQ:INTCC), Facebook Inc. (NASDAQ:FBC), Alphabet Inc.’s (NASDAQ:GOOGB) Google, PayPal Holdings Inc. (NASDAQ:PYPLC), Amazon.com (NASDAQ:AMZNC), Microsoft Corp. (NASDAQ:MSFTC) and McDonalds Corp. (NYSE:MCDC) to name a few.