I'm in the same boat as the OP. I have an iRig and was about to record line-in, but it will only be in mono. Bleh.
I knew that GB supported midi in for keyboard, but it does drums now too?
I'm gonna have to try this...
Midi is the same from all devices. It is simply computer data that communicates a musical note with various parameters (pitch bend, modulation, etc). An eDrum set module is assigning a specific note to each drum, which means when you connect and hit the snare, the module will send an "A 440Hz Note" which then the computer or iPad triggers that note in whatever instrument is digitally selected. Therefore if you have the keyboard open in GarageBand and use an eDrum set, you will play piano notes when you hit the drums. Midi drum patches by default usually fall under the same note scheme. Certain notes are typically used for the same type of drum, I.e. a middle C Note will always trigger a kick drum (just a fictitious example).
^let me know what you find out...
i don't need two lines in at once...just one.
So i'd record the guitar or drums first, then go back and record whichever i didn't record. So it's two separate tracks, but recorded at separate times.
If you are okay with mono then your best bet is probably the Apogee Jam right now. The iRig and similar devices that use the mic input are quite noisy. The Apogee jam is designed for instrument level signals which is between mic and line. Your DM5 outputs line level if I'm not mistaken, but the gain on the Apogee can be turned down for the perfect input volume.
FYI I owned a DM5 for several years and used it to record using both audio and midi. I ultimately sold it because I couldn't stand the low polyphony and lack of variable hi-hat. My point is that I can help if you have any other dm5 questions.