That hearth is a beautiful place to start! Is he of Celtic heritage? Does he have memorabilia that can be displayed? Family photos? Artifacts, etc? I would collect those and display them on those shelves. Rich, saturated colors…dark mossy greens, blues, deep red, rich brown are all colors associated with Ireland and Medieval culture. I can imagine one side of your fireplace area being devoted to a bar. Perhaps a mini fridge stocked with beers, Guinness, etc. And a gorgeous silver platter with Waterford crystals, a snifter for brandy, etc. on display. A deep, comfortable sofa with rolled arms. Perhaps a leather chair in one of those dark, rich colors, with an ottoman and a little side table with a lamp on it. Think of creating a cozy atmosphere. A woven rug with, perhaps a Celtic knot pattern woven into it. You don’t want it to be kitschy. Rather, rich, warm, cozy. Good luck with your project and keep us posted!

Dark

What if you stain the pine>

You have a good start with the stone and brick fireplace. No wallpaper. Don't paint the shelves. Depending on what he wants to put on those shelves and the height needed between shelves and the budget, I'd use much thicker wood for shelves. What's there looks thin and modern, although the black iron brackets are good. Stain the shelves and wood behind them a dark color.

Maybe a medieval style tapestry on the wall. Here's one example.

Good question about the rug. Since we're not doing straw or rush mats in homes :-) I'd either go with additional animal skin floor coverings or, my preference, a darkly colored real or machine woven oriental style rug, with dark, saturated colors, not the washed out/distressed look popular today.

Comfortable leather easy chair and sofa in a dark leather. If he likes, taxidermy on the wall.

This fellow built himself a dungeons and dragons fantasy room. There's a lot of kitsch, but you might get some usable ideas. Link to article.

I don't know how to reply to each of you personally. HUGE thank you for your input. I am loving seeing this room come alive though different eyes.

Should’ve made the whole area in stone with shelves having the claddagh weave

Hi Claire - so interesting about the shelves having the Claddagh weave - do you mean on the edge or the actual entire shelf?

What does the man want in his room? Does he prefer wallpaper or paint? Does he have belongings he would like in his room to decorate it? Does he have a rug preference? If he has weapons, armor, hearldry items, hangers meant for hanging them on the walls for display can be nice. Is the fireplace functional? If it's wood burning, a wood rack and fireplace tools can be handy to have there.

The edges

I like petreckel’s “rich, warm cozy” idea. Here’s a couple of suggestions for accessories.

Is he into Gaelic things or Celtic things? It might not make much of a difference to some folks, but if he is really into it, he might know the differences and it may or may not be a thing for him. Might also want to find out when during medieval he is into- if he's into something more 400's vs 1400's is a big difference.

WOW! Love the throw. I was thinking of something rich and warm like that over the couch.

Such a valid point 400's vs 1400's - thank you.

This is my first time jumping in here asking for feedback and advice and I am blown away by the generosity and insights.

It is a very nice throw :) Does his name begin with C?

No with a D :-)

Maybe a throw with a monogram of a D instead of a C then for him. Or a throw without a monogram :)