A red dot gets you on target faster than irons alone, but a few choices up front make a big difference. Start with dot size: a small dot, around 2 MOA, covers roughly two inches at 100 yards and suits slower, precise shots. A larger dot is easier to pick up quickly at close range.
If the rifle already has flip-up backup sights, think about co-witness. With absolute co-witness the irons line up in the center of the optic window; with lower-third they sit at the bottom and leave more of the window clear. The height of the optic mount decides which one you end up with.
Once it's mounted, torque the screws to spec, add a little thread locker, and re-check everything after the first range session. If you're looking for an optic to run over backup irons, take a look at the Ozark Armament Rhino red dot sight.
Not sure which sight fits your build? A good place to start is Ozark Armament on Amazon, where the company keeps its AR-15 iron sights together. Read each listing's mounting details before ordering.