Things you need:
Iris Manager:
http://trueirisdownloads.blogspot.com/2013/01/ps3-homebrew-iris-manager.html
TrueAncestor SELF resigner (only for the required files)
http://www.mediafire.com/?4ojs79teb4486x4
A file manager (i.e. multiMAN , Comgenie Awesome FileManager) OR a working PC-PS3 ftp connection
Install the Iris PKG on your PS3 (The pkg is inside the rar file: irismanager-4-x.pkg)
1. Extract the Iris Manager rar file on your PC
2. Inside the Iris uncompressed folder, you will se a file named patch_ps1emu.rar. Extract this file
3. Extract TrueAncestor SELF resigner and copy the contents of the DATA folder (ldr_curves, vsh_curves, keys) to the patch_ps1emu DATA folder.
4. Copy the file /dev_flash/ps1emu/ps1_emu.self (using FTP or a file Manager) and put this file into the patch_ps1emu extracted folder
5. Run patch_ps1emu.bat with Administrator privileges
6. Copy the generated ps1_emu_patched2.self to the IRIS MANAGER folder on PS3 (usually /dev_hdd0/game/IMANAGER4)
Example: /dev_hdd0/game/IMANAGER4/ ps1_emu_patched2.self
Preparing your PSX ISOS:
On your EXTERNAL USB device, create a folder called “PSXISOS”
/dev_usb000/PSXGAMES/
Inside the PSXGAMES folder, create a foder with the game name
Example:
/dev_usb000/PSXGAMES/CrashBandicoot/
Inside the game folder, put your ISO/BIN/IMG and Optionally, a JPG image named “Cover.jpg” (to get the cover showing on Iris Manager game list),
The Directory structure should be like this:
/dev_usb000/PSXGAMES/CrashBandicoot/CrashBandicoot.iso
If the game has multiple disks, put the other disk images in the same folder respecting the filename of the first disc. Example
/dev_usb000/PSXGAMES/FinalFantasy9/ FF9_disc1.ISO
/dev_usb000/PSXGAMES/FinalFantasy9/ FF9_disc2.ISO
/dev_usb000/PSXGAMES/FinalFantasy9/ FF9_disc3.ISO
/dev_usb000/PSXGAMES/FinalFantasy9/ FF9_disc4.ISO
The Cover image must not exceed 1024×1024 in size
Done. Your PSX games will now apperar on Iris Game list
Note: there is a pkg containing the patched ps1_emu_patched2.self , you can download it HERE if you are having problems patching your own file. Im not recommending using it, since i have not tested this pkg myself.
Example of the correct folder structure using Iris Manager:

PS3 GAMES:
Iris Manager will search for your PS3 games on /dev_usb000/GAMEZ instead of GAMES
If you are a multiMAN user, you can just rename your GAMES folder to GAMEZ. Since multiMAN search for games in both GAMES and GAMEZ folder, you will have no problems.
GAME COVERS
You can use the same multiMAN “covers” on Iris Manager! Just copy the “covers” folder into the Iris Manager directory
Ex: /dev_hdd0/game/IMANAGER4/covers
Tips:
To load the last played game: Hold L1 at Iris Manager startup
Sorting the games by system: Press L2 (PSX) or R2 (PS3)
Game config: Select your PSX/PS3 game and press SELECT
General config: Press START
Iris Manager Translations:
brunolee has provided a nice language pkgs, you can get these translations
here: http://trueirisdownloads.blogspot.com/2013/01/ps3-homebrew-iris-manager-packages.html
Just install the respective .pkg and you are done!
Have fun!