My late wife could write mirror writing just as easily as regular. She was right handed and did not have any other similar skills.
I'm right handed, and whenever I try to write with my left hand it comes out as a mirror image of what is should be. I can write something with both hands at the same time and the left comes our as a mirror of the right.
I saw that done on TV years back (a show called "That's Incredible") so I tried it... and there it was. They said something like 5% of all people have that capability.