If you can unlock it, it's very easy as the mounting bolts are uncovered when it's unlocked.
If it is locked, then it's a bit more difficult but not much more. The lock housing you can see in the image below is just stamped metal riveted together. Some ring locks are so weak that a good whack on the locking lever will spring it open. Otherwise, the easiest way to remove it is the same way thieves would do it: use an angle grinder or bolt cutters. It takes about 10-20 seconds with the right tools.
Because ring locks only lock the rear wheel to the frame, they are very low security. They're fine for low-theft environments and for heavy steel utility bikes or mommy-bikes, but if you have a nice, light bike, it's easy for thief to just pick up the bike and throw it over their shoulder or throw it in a truck. Essentially, ring locks keep people honest in honest communities.
Source: I've lived in Japan for several years where ring locks are the main style used, and had to replace locks for family members who have lost keys.