They already have low paid customer service and ops in Ireland. They won't get engineers to move to there. Like finance guys and the promise of tax free living in Dubai - at the end of the day they don't twant o be in a backwater.
Let's not forget that there are many engineers who don't live in the Bay Area, NYC or London, for whom Dublin may be quite attractive. And while it may not be on the level of those three regions, it has a lot going for it in its' own right.
Now Ireland has both: human capital and tax reasons.
Not saying there aren't plenty IT jobs, but you can't look at size/investment alone without looking at what kind of roles are actually there.
I'd also note that while companies get plenty tax breaks, actual income-tax is pretty high. Accommodation is pretty pricey near the centre too, though not as bad as London.
Dublin in particular is incredibly dynamic right now. And it's a lot better than where most London tech companies are e.g. Canary Wharf, Stockley Park etc.