Dublin is by no means a 'backwater' - Google and Facebook's EU headquarters both sit in Dublin, as well as large outposts of a number of global financial institutions. There's a lot going on in Dublin, not to speak of its history, accessibility, and affordability compared to the canonical 'tech hubs'.
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.
They do need the HQ for tax reasons but they wouldn't need to hire that many people (more than in London for example). They do that because they want to.
I work in IT in Dublin. There is a growing IT sector, but the larger companies here are a lot of support and infrastructure (Goog Amz etc).
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.
Whatever "formally" means, many US companies (Facebook and Microsoft are the best known examples) attribute their european earning to Ireland in order to enjoy a low tax bracket.
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.