Irrlicht.Net is no longer developed or supported, Irrlicht.Net Cross Platform is a much more complete wrapper. Please ask your C# related questions on their forums first.
We are using Irrlicht.NET in our products that is developed in .NET with a lot of WinForm GUI including menu strip and toolstrip. I am not sure how do you implement your main rendering loop, but we use a multithreading approach and the framerate was pretty consistent.