I'm currently modeling terrain in Maya and exporting in .obj files. The finished product looks pretty good in Irrlicht.
I'm not seeing a problem with polys yet really. I'm building huge tiles with houses and roads and stuff as minutely detailed as a I can, and it's still about only 30k for a mesh that almost makes a horizon

I actually have the draw distance set to 20k right now I think, and there's still map after that. That is, though, without trees, I've not decided about how to do foliage yet. I want wind...
So yeah, Irrlicht can handle it, it seems, and most computers probably can as well.
I'm running a 2.4 Celeron, 4 gigs DDR ram and a 256 meg Radeon 9600 running at about a gigahertz. I get about 40 frames running with OpenGL.