a story of patriotism...

He was in the wrong, simple as that.
He wasn't arrested for exercising his right to protest, neither was he arrested for waving his flag.
He was arrested because he chose to sit in the public section of a meeting but choosing to not follow the rules that come with that choice. In fact as he was arrested for not following the rules after he had been warned that he was breaking the rules it means he was actively seeking the outcome that he got which Darkside finds so outrageous.

So the claim of.....

What in the devils name will become of this country if we can't even raise our national flag anymore?!

....is complete rubbish.

Now compare this with the US, rarely do I see any pic without the US flag showing up someplace.

Try waving a flag in a public gallery of a political meeting in the US where there are bans on unauthorised demonstrations in chamber.
Try doing it after you have been told to stop.

Today my dad was fined with €100.
I don't know yet if he will pay it or take legal actions.

Legal actions for what?
He broke the law.