The double Irish arrangement isn't a loophole, or if it is its a loophole that's the fault of the Irish by not taking account of US transfer pricing rules. The basic problem is that transfer payments within multinational corporations makes it hard to account for income, and rules to attempt to achieve that accounting are easily undermined by tax haven jurisdictions.

The double-Irish is definitely a loophole. The basic premise of the loophole is that Irish-incorporated companies don't owe Irish income taxes on their business activities if they are not managed in Ireland.

This is a relatively recent change to the Irish tax code (based on the pace of tax law development)--and was introduced only after the development of transfer pricing rules (assignment of income to/between subsidiaries of multinational companies). In other words--the loophole was deliberately designed to get around U.S. and European transfer pricing rules and make Ireland "competitive" for multinational corporations.