What is that shingle-like material used on the outside of older houses, and is it dangerous?

I own an old home, built during WWII. It was originally intended as temporary housing, but was never replaced. So, of course, they did not use the best materials. It is wood-frame, and originally the outside material was a kind of board that looks like roof shingles, but with that gross brick-like pattern that fools no one.

There were two additions to the house, built onto the back, and one is a sun room. I want to open up the walls, by stripping off the paneling, and the paneling and old gypsum board it is over on the other side of the wall, to reveal the skeleton of that wall. I want to do this to improve airflow and heat/cool-air distribution. However, I pulled back the paneling in the sun room, on what was once the outside wall, and discovered that some, but not all, of the cladding, or whatever you call that material, was removed, it was just paneled over. So, I have three questions RE this:

First, what is that material called -- the cladding on the outside of a house, that looks life roof shingles?

Second, is this material dangerous? Does it contain, say, asbestos, or is it safe for me to remove on my own, without any special precautions?

Third, if it is dangerous, because it contains asbestos, or something like it, how bad is it to have it sitting between what is now an inside wall? ( I have cut a legal-paper size hole to reveal what is going on.)