Several modelers can directly export .bullet files, such as Blender, Maya, Cinema 4D etc.
In the Bullet sdk there are various ways of importing files, such as
.bullet, see Bullet/Demos/SerializeDemo
.obj, see Bullet/Demos/ConvexDecompositionDemo
.bsp (Quake/Doom), see Bullet/Demos/BspDemo
Once you have the mesh data in memory, it should be easy to create a Bullet collision shape from it.
There are also various examples how to integrate Irrlicht with Bullet. Check-out the branch of IrrKit:http://code.google.com/p/gamekit/source ... s%2FIrrKit
(note that the trunk uses Ogre, but that branch uses Irrlicht)
In other words, no need to re-invent any wheel there.