This is port of Mednafen PSX core to the libretro API. It currently runs on Linux, OSX and possibly Windows.
To run this core, the "system directory" must be defined if running in RetroArch. Here, the PSX BIOSes must be placed, $sysdir/SCPH550{0,1,2} for Japanese, NA and EU regions respectively. Memory cards will also be saved to this system directory.
Mednafen differs from other PS1 emulators in that it reads a .cue sheet that points to an .iso/.bin whatever. If you have e.g. foo.iso, you should create a foo.cue, and fill this in:
FILE "foo.iso" BINARY
TRACK 01 MODE1/2352
INDEX 01 00:00:00
After that, you can load the foo.cue file as a ROM. Note that this is a dirty hack and will not work on all games. Ideally, make sure to use rips that have cue-sheets.