From my basic understanding of RTEMS, they are similar on the surface, but Ironclad has a wholly different set of aims.
While RTEMS aims to provide an extremely lightweight and easily embeddedable system (and thus it forgoes things like per-process address spaces, which increase footprint and needed hardware), Ironclad puts a really strong focus on security, with things like mandatory access control, full user support, capabilities, and other goodies, sometimes at the expense of those things. Its more of a QNX and RTLinux than an RTEMS.