It's not a fine. The EU is saying: "You know those taxes you didn't pay and Ireland looked the other way because it benefitted them? Ireland couldn't look the other way because they signed an agreement with 25+ other countries to not do that, so be a good sport and pay all those taxes we told you to".

"I'm not making you pay me. My boss is making you pay me. So don't be mad at me, be mad at my boss while I get your money."

Ireland fought against this decision because it's a short term win, but potentially a massive long term loss.