Have you tried making one yet? Something like this....
Code:
mkinitrd /boot/initrd-2.6.13-1.1398-xenU `uname -r`
# mkinitrd --help
usage: mkinitrd [--version] [-v] [-f] [--preload <module>]
[--omit-scsi-modules] [--omit-raid-modules] [--omit-lvm-modules]
[--with=<module>] [--image-version] [--fstab=<fstab>] [--nocompress]
[--builtin=<module>] [--nopivot] <initrd-image> <kernel-version>
(ex: mkinitrd /boot/initrd-2.2.5-15.img 2.2.5-15)