But how I am able to save data in PS2's Classics Placeholder, since i'm mounting games on IRISman, and there's no Memory cards button here?
Use the File Manager to manage your save data files.
Or create standalone icons for each game using PS2 Classics GUI to create the packages.
Unlike the PS2 Placeholder that share the save data file with multiple games. If you create an icon for each game, the save data file will be exclusive for that game. Additionally you don't require a backup manager to mount the game.