Argh!!!!!! Seriously????

Razor scraper and elbow grease.

After you get it off what are you planning to do for that area?

Not sure what you goal is, but no matter if you get the mastic and paint off, the old painted brick will never look like the new brick, so it's safe to assume that you'll end up placing something over top.

I checked for mirror mastic and evidently they make a product to remove mastic

We chiseled without dry ice and most of it came off, but there are definitely still remnants. Tomorrow we'll do the razor scraping and maybe some sanding. The goal is to expose the brick. We were tired of the late 80's/early 90's white painted brick with mirror over the fireplace look. I've read about a stain that actually restores the red of the brick to its more natural color. But even if we just get this sort of antiqued look, I'm ok with that. We are doing light grey walls, dark floors. My inspiration pic:

So you already stripped paint off the rest of the brick? If so once you have the chunks of mastic off use the same stripper. The stripper 'might' melt the paint under the mastic releasing everything.

I tried a test patch! Let's test the theory!

I see where you are going with your inspiration pic - thanks for the image- it helps a lot.

I still would like all the brick to have the same finish and I think that may be hard to achieve because the insert brick is painted and the surrounding brick is not.

I think using those stains on brick just end up looking like stained brick.

I'm not sure but I'm thinking it might be best to paint the surrounding brick white and then when you have all the surfaces white, start the scraping off/removal process in order to get the finish you desire as pictured in your inspiration photo.

All of the brick was the same white. There was a mirror covering this last portion of white. We stripped the rest then had the mirror removed and now we are stripping the last piece. So they should all be exactly the same. They have a washed/faded sort of greyed out look that I can't tell if I like. Once I get the last of the paint off, I'm going to try one last cleaning with the stripper or maybe just with some baking soda to see if they will brighten up a bit

True a course wire brush on a drill. You could try a hand held one but it doesn't work as well. Lots of dust and should get it right off especially if you use knotted version on angle grinder.

So the best outcome was where we just did chiseling and sanding. The paint stripper did help to remove the mastic where but also seemed to blend with the stripper and stained the brick a bit. There were spots between the brick that we couldn't do a 'dry removal' on and those came off with paint stripper. But because it was between the bricks, the stains aren't noticeable. The only stains that are noticeable is where I tried to do a 'wet removal' by soaking it in paint stripper and softening it on the surface of the brick. I hope that makes sense to anyone reading this who might face the same problem!

Bottom line: dry removal by chipping and sanding.

See the two spots over the arch? Those are the mastic/paint thinner stains. I doubt they will be noticeable after I seal the brick.

If I was doing this project, I would have someone come in and soda blast all the brick. The process will be a bit messy, but it's just baking soda. I have a home built in 1840 with cream city brick, it's a very soft white brick, anyway we soda blasted the whole wall and removed all the paint ! It really is the way to go....

I don't know why we don't have these sorts of things in Southern California. Someone else mentioned dry ice cleaning and I couldn't find that. All we have is sandblasting. Everyone just loves painted brick here I guess!!

Soda blasting is very similar to sand blasting. Only difference is that baking soda is used instead. It a much less aggressive way to remove paint and such. We had to hold the gun sideways to bast all the brickwork because if you would point it directly at the brick it was too aggressive and would tear everything up.
Yours looks good though! I'm sure it was a lot of work.