Also it is (and was) the only English speaking country in the eurozone.
Tax was a huge factor in drawing some of the big names but the double-Irish (the worst aspect of tax avoidance) had been phased out. Corporate tax is 12.5% which is lower than the majority of EU countries [1] but the existing ecosystem of companies has also helped create an attractive labour pool. If it's purely tax everyone would move to Hungary (9%).
A number of European countries have near 100% English literacy and allow for document workflow and accounting in English. I suspect it's mostly the taxes.
I totally agree that English can be used entirely for businesses in other euro countries but for a US company it is attractive for employees/families to be in country where English is an official language.
> The EF EPI calculates a country’s/region's average adult English skill level using data from three different versions of the EF SET. Two versions are open to any Internet user for free. The third is an online placement test used by EF during the enrollment process for English courses.
> In order to calculate a country’s EF EPI score, each test score was normalized to obtain the percentage of correct answers for that test. All the scores for a country/region were then averaged across the three tests, giving equal weight to each test. Regional and global averages were weighted by the populations of each country/region within each region.
People who don't know english in the first place won't have any reason for taking those tests.
That's due to tax reasons. US companies extracting value from Europe while avoiding taxes at all costs.