And one of the typical ways we refer to people being an idiot in both Irish and English in Ireland is to describe them as an 'amadán'.
Can it be used both affectionately and in genuine annoyance in Irish as in English?
Well, that says more about your background than it does the prevalence of the term. Dubliner, I'm guessing?
Close enough: that puts you in the Pale, so Irish was dead for centuries where you're from. If you go further out into the country, you'll here more terms from Irish causally used in conversation in English. Your background means you're significantly less likely to have heard words such as 'amadán' in daily use amongst English speakers than if you'd been brought up in, say, Monaghan.
I'm from Drogheda my self, but me ma is a Dub and would have said amadán to me a lot, especially her parents. I can tell you that the accusations were groundless
I typed 'silly' for amaideach in this case but would/should foolish also be considered acceptable too?
No. When you are linking a noun (buachaill) with a pronoun (é) you must use the copula - "Is buachaill amaideach é"