As long as you have picatinny rails, there are an assortment of flip up sights you can attach them to. Do your research, and see which ones fit your needs and likes.

It really depends on what you plan on running as your main optic. If your running a reddot with a quick detach (qd) mount, or a reddot that can cowitness then id go with the magpul Mbus or Mbus pro (depending on how much you want to spend) Troy Micro Sights are good too.
If your running a fixed power (like an acog, or prism) or an LPVO then cowitness isn't really possible/practical and you'd be better off running offset sights, or no irons at all
ADE brand offsets here
amazon.com/Front-Degree-Rapid-Transition-Backup/dp/B00EA7XDTO

some KAC brand offset here
opticsplanet.com/knight-s-armament-45-degree-offset-folding-sight-set-clamp-mount-600-meter-micro.html
Here is a quick video that shows the offset irons how they work
https://youtu.be/BuGfei28kSw

Practically all flip-up sights work with Radical 556. I recommend Magpul MBUS, Troy, and FAB Defense Flip-Up Sights. CHECK HERE ( midwayusa.com/product/2319289991 ) and ( midwayusa.com/product/2319221264 ) I hope this helps!