I would like to record the licks I play while exercising over a metronome in order to improve my timing. I need a way to record both the lick and the metronome beats to see if I'm picking the note exactly on the beat. Can I do something like that using iRig and Garage Band? And can you give me some hint? The metronome could be replaced by a drum track or something like that.
Yes, you can do that with iRig and GarageBand. It'd be easiest to just forget the (external) metronome and use the (internal) one in GarageBand. In other words, you don't have to record the metronome, it's built in as part of the recording process. So you just turn it on before you record and then once your guitar part is recorded you can toggle the click on or off as you please.
And yes you could use a drum loop in GarageBand instead.