Ireland doesn't have the topography to make it work at a large scale...
Low-head hydro actually has major advantages over the common megadam type we see all over the American west. Little capital investment (relatively speaking), less environmental impact (no monstrous impoundment lakes, no habitat destruction, no communities forcibly relocated), less effect from annual rainfall fluctuations, and less economic impact if a generator plant is offline for a while due to a local drought.
My guess is that corporate rates are likely higher than residential rates and that, combined with the larger electric draw
Industrial customers pay demand rates -- the higher their draw, the higher the cost per kWh (in stepped tiers, like the income tax rates in the US), and the lowest tier is already a higher rate than residential, because they're also charged for their installed capacity (that is, a 5000 A service pays a higher rate than 500 A, even if actual usage rarely exceeds 500 A -- common home systems are 200A to 500A). Even if the solar installation can only cover enough of their peak load to avoid a tier jump, they'll make back the investment before the cells start to deteriorate significantly (even a factory like that probably doesn't run three shifts, so peak demand is generally daylight hours). Not to mention, like many things, you pay less per panel if you're buying 500 of them than if you buy 5.
All of these things are probably also true in Germany, just with different numbers attached.