Ah, Irrlicht is AWESOME (best 3D engine, except maybe Ogre [but simpler]).
However, there is no easy way to do this. Two options exist:
1. Convert glest's models to another format (3DS and X work best with Irrlicht). However, there aren't any ways to convert animations. You see, the g3d importer only imports the still model. Its an open format though, and if you want to take the time, you probably COULD make a better importer or converter. All of G3D's techniques are pretty standard, and the importer and exporter may help. (They're in python, and can be downloaded via the glest.org download page).
2. Do some heavy coding to allow your program to use G3D. This probably wouldn't be easy, but not impossible, since G3D is an open format. If you'd study the source code to see how Glest renders and uses G3D, then you may be able to slowly adapt that code into your game.
Neither method is easy, but glest DOES have very nice models and so few restrictions... All up to you.