It's for the entire EU, not just Ireland.

I believe technically it will be Ireland collecting, since the case was started by the local authority (DPC) against a local company (WhatsApp Ireland). European authorities were only brought in to determine what the scale of the fine should be, since there was no clear consensus (initial suggestions were around E50m, which is why WA put aside a safer E77m). But yes, the fine was determined looking at the entire EU userbase, which makes it still peanuts in the great scheme of things.

Who is collecting isn't relevant to what the per-user cost to Facebook is though? The other EU countries won't fine for the same thing (as the poster suggested they would) is the point, so doesn't make much sense to relate it to the number of inhabitants of Ireland.