You could use the RCA outs on the UA5 to the iRiver, but the UA5 doesn't have true passthrough. What that means is that the signal from the mics will go to the UA5, and then be converted to a digital signal, and then be converted back to analog before hitting the RCA outs. While not a horrible thing, most people would probably prefer to record digitally to their recorder, or have a pass through, that skips the A-D and D-A all together. Hope that helps.
PS: You can get a Stereo RCA -> 1/8" cable at radio shack