the safe way to do things with x format, is one I know, but is not necesarily the most powerful. I tend to deal with nitty gritty when at a project, but when giving a quick advice, I only recommend the safe(you with further trial, and research, will get more) :
-try every mesh be a single mesh, or attached/combined/grouped...depends on the 3d package used.X tend to be preferred by engines as single unique mesh.
-try also to apply to it a simple, standard material, one texture (yup, can be done: I even used to make ONE texture for a whole level overloaded of objects, just for engine optimization reasons, 3d is more flexible than people think
) to you single mesh.
-Get informed on what exactly is supported in x format. get informed on what of x format does actually support Irrlicht. (ie, transparency, environment, etc...I dunno as I don't code and not involved in any irrlicht project)
-Remember blender has several flavours of x exporters. Not all work the same. To date, I think there's now Ben's 2.41 recnetly done one, a dx9 flavour made by another guy (which based in an older version, unfortunately, though) , a dx7 one(probaly better suited for multipart objects, but I dunno ).
-Beware, if not used much Blender, or 3d in general, you couldbe having double faces in some area of your model. If u have two tris overlapping, it will add weird visualizing tricks. Not sure but think was the prob in your first shot. Just check. Do a remove doubles in Blender or something in that line. I don't tend to use Blender for modelling, but there are several ways in it. And indeed, is easy to end up doing the artifcat in Blender, accidentally.
Every tool out there has a way to fix that.