There is a couple things you can do:
1. Build a 40" high half wall using typical 2x4's and drywall. This will cover the two openings.Put a 16" wide granite bar top for seating. 1" overhang on the kitchen side will leave you with about 11" overhang allowing ample knee space. I expect the length of the existing cabinets are 48" so the top will be 50". Only room for two stools on this plan. No additional support required.
2. One way to get seating for three is to make the bar top the shape of a half moon. This is nice because it's more comfortable to talk compared to diner style seating. Your granite fabricator will provide a flat steel support that is mounted to the top of the wall and extends to support the overhang.
I would suggest replacing all of the counter tops in granite, quartz, or solid surface when you do this.

Additional supports will be needed. I suggest you pull off the face frame and put a plywood sheet over those openings, that goes from the underside of the counter all the way down to the floor. Veneer over it. Then, you could probably run 4 triangulated L brackets. I suggest 8" brackets min. You'll probably want an overhang of 15" to the high point of the curve, to provide enough room for knees at the ends of the curve. if you go straight across, you could do 12" for the overhang min. Some say 10" is the min but I'm short and that isn't even enough for me.
Just guessing at the dimensions, your countertop would need a seam and the seam should split the curve. The granite guy can add the brackets, but won't always want to. I used to do both cabinet work and stone work. Either one can do it. Sometimes, depending on the company, the stone guys just want to put their counters down, install sink/cooktop, and go.