Apple did a deal with the ROI Government so that they woiuld pay only 2% (I think) corporation tax on Apple's profits from all its EU operations by chanelling all its EU generated revenue into Ireland.
I believe Amazon did the same with Luxembourg.
It was one reason why I was getting sick of the EU. It was allowing large International companies to avoid massive amounts of tax, especially when poverty was increasing in the UK partly because of cuts in public expenditure, because of reduced tax takes.
Eire has benefitted a lot from EU funding, because of its rural economy. 40% of the EUs spending is on agriculture. In the past Eire also benefitted a lot from EU regional aid although that has reduced in recent years, because Eire is no longer poor. This funding has allowed Irish farmers to be more efficient. The roads in Eire used to be poor, but not now.
40 years ago many young people from Eire came to the UK to work, now it is the other way round with people going from the UK to Eire to work. There are some big Irish companies in the UK now such as Kingspan (building insulation) Kerry Foods, Associated British! Foods (who own Primark). Magners, Smurfit (Packaging) , Paddy Power/Betfair.