It is an national accounting reason, rather than the Irish people have higher incomes or being more productive
It seems that some companies previously deemed to be foreign are now deemed to be Irish, and so their profits now count towards Irish GDP (and, to the extent they are not a subsidiary of a foreign company and retain their profits, also to Irish Gross National Income). The Irish Central Statistics Office said a 26.3% increase in GDP and 18.7% in GNI from 2014 to 2015 at constant prices
What may have happened is inversion where United States companies are notionally taken-over by Irish companies (though the old US shareholders remain shareholders of the new merged company), largely to take advantage of Ireland's generous corporation tax regime. That makes their production/value-added/profits appear to be Irish this year, boosting Ireland's GDP compared with last year
Further reading from 10 months ago when this was noticed: