Put a false front and sides on it floor to ceiling in MDF snd then paint it. It will look like it is plaster.

I think it looks okay. Lots of fun ways to make a feature of it.

But if you really can't stand it just remove it and plasterboard over the hole.

I was going to suggest a big plant on the floor to the right of the opening & see that Sonia has done the same. I actually think it won’t be as noticeable once the room is furnished either.

Remove the mantel surround, it’ll look much better! You can cover up with an offcut of plasterboard but personally i’d clean up the firebox, add some tiles or paint and store some nick nacks in there.

On zooming in it looks like the mantel is separate? It looked kinda cut off at one end. It might look much better just with the surround, which looks to have decorative corner pieces. So although it's off centre, taking away the mantel may lessen the visual impact but still keep the decorative element for you to enjoy and play with.

I too would remove the mantel if possible and either do without or add a new one, symmetrical with the surround. I would put in a plasterboard wall on the left, from floor to the level of the top of the chimney opening (black area). Install shelves above in wood or paint in a deep colour - the horizontals will optically widen the area and give a more balanced appearance. When the skirting board is reinstated it will look altogether more pleasing.

I’m guessing you won’t actually use the fireplace but here’s a good example of something you could do - the log store in the picture gives it balance. Maybe also replace the mantle with some nice oak and extend it across the full width, rather than remove. Possibly then shelve below and add favourite ornaments, photos books etc. Unless you are very minimalist I think it would be a shame to plasterboard it in.

It may have been built that way if it shares the chimney breast with another fireplace, personally I think it looks fine and perhaps I'd just balance it a bit with something to lead the eye on the wider side, a tall vase of dried flowers, put up a hook and hang some fireplace tools, a poker, tongs or whatever, or even a small artwork. I like its original look.

You haven't said whether you own the property or are just renting. It will make a huge difference to what is feasible.

Check whether it is all fireplace- one of ours is off centre and it’s because the pipes connecting the hot water system have been put up it to take them upstairs and then boxed around to hide them- when we get a new heating system we can take it away and it’ll look better

Just add shelves in left hand alcove top to bottom you won’t notice it then.

I have something similar and have balanced it with plants! For your particular dilemma I would personally get a relatively mature devils ivy (or any type of training pothos or philondendron plant) so it trails down to the right of the fireplace (assuming its non-functional also!). They are super easy care plants too, which is a bonus :)