80% of all corporate tax paid in 2015 came from non-irish businesses. 40% of all corporate tax paid in 2015 came from 10 companies. Apple are the largest single taxpayer in Ireland. (http://www.revenue.ie/en/about/publications/corporation-tax-...)
So the Irish strategy is to be nice to multi-nationals, in return for that 80%. The understanding is that if we taxed more, they'd move somewhere else and we'd actually receive less. e.g., biting the hand that feeds you.
(This is also why the Irish govt are going to defend Apple in this - this isn't just about Apple for them, it's about scaring off 80% of corporate revenues)