I'd say it depends on the things you want to record. For rock concerts and loud environments, you'd need at least a battery box. For very quiet recordings you need a pre-amp because you'll hear the ADC in the iriver gets noisy above 15dB (or even below that).
For everything else, just try it plugged like that, the iriver provides around 3.7V on the plug, most mics are happy with that.