To boot via UEFI (not CSM\MBR\Legacy boot) you need two things (to start with)...
a) A FAT partition (could be in a GPT partitioned disk or just a plain FAT/FAT16/FAT32 MBR style partition)
b) A correctly named and positioned .EFI boot file which matches the UEFI implementation
e.g. for Intel x86 32-bit UEFI firmware (note that this is not necessarily a 32-bit
CPU, as a system containing a 64-bit CPU may contain 32-bit UEFI firmware) - there MUST be a
\EFI\boot\bootia32.efi file on a FAT partition.
For an x86 64-bit UEFI firmware implementation, there MUST be a
\EFI\boot\bootx64.efi file on the FAT partition.
Typically, Hirens does not contain these files and cannot UEFI-boot. Unless you have a special version which contains one or both of these .EFI files, you can only boot Hirens via the old MBR\CSM\Legacy route.
This may be useful.