Missing Eyefinity support on RHEL 5 is a known issue.
When Eyefinity was planned/implemented for Linux, it only included "modern" OSes with RandR 1.2 in the scope.
RHEL 6 has RandR 1.2 and works with Eyefinity.
We're not aware of any installation problems of our driver on RHEL 5.5.
Could you provide output from "atigetsysteminfo.sh" after a stock install (without patching firegl_public.c) so that we can see what's wrong.