i am not able to give you a support of how to set up the gamma or brightness with irrlicht functions. one possibility iss to set up the brightness or gamma by your driver but I don't think that is a sufficient solution for you . I am yet not experienced enough in irrlicht to help you
q|^.^|p beeing every time friendly to everyone
sys: 2500+Barton 512MB 6600GT WinXP
Well if you are getting different levels of brightness between different PC's, and not the same, that would suggest the problem is with the PC's and not Irrlicht, wouldn't it?
I can tell you, on my computer, the maps look fine.