Yeah, I agree that cultural norms vary a lot depending on the culture. For the Celts, in particular, there's a lot of guess work as there were a lot of tribes, spread over a vast area, over a vast period of time. Cultural norms definitely varied from tribe to tribe (and over the decades), and there's not a huge amount of information to base anything off of. (I'd probably start with old Celtic legends like the Tain, but...that's definitely written within the Christian era, etc.) But you can still get somewhere...

I have a hard time with historical fiction too, as so much of it is really, really bad. I tend to prefer fantasy which is a bit more gritty and realistic, too. (With less magic, since as you said, it tends to be used more in a deus ex machina way than in a way that really works for the world and culture.) But, eh, at least I can go "fine, I'll suspend disbelief", while I really can't do that in a story that claims to be from, say, Classical Rome, yet has a Roman lady who is all "Slavery is horrible and I am a modern liberated woman with modern concepts of self-esteem!" (And in a particularly bad case, is a princess who wears a purple velvet ball gown.)