Thanks for weighing in SGG!
OK, I'm not overly concerned with generating virtual sounds from Sampletank (this app was suggested by the vendor to verify that the keyboard was recognized by the IRIG, which it is), nor even from Garage Band, as I like to play everything myself (guitars, bass, keyboards, vocals, etc). HOWEVER YES, I WOULD like to utilize the many "voices" resident in the TG55 (to generate sounds from that as you suspected).
So while these may be considered "virtual sounds" I've been playing a standard 88 keyboard for many years, whether a grand piano, upright, etc so I'm very comfortable with the Yamaha CLP760 keyboard, utilizing the voices inherent in the TG55. When I plug the keyboard into the TG (along with audio out cables from the TG into the keyboard, which has excellent stereo speaker sound) it sounds great!
I'm going to try and connect the keyboard's MIDI OUT to the IRIG MIDI IN (which I've done and Sampletank "hears" that (not sure about GB but will try again) but then the TG is left out in the cold. So your next step is to connect IRIG's MIDI OUT to the TG's MIDI IN, right?
In the end, my MAIN goal is to be able to record songs on keyboards (I can already record guitar, bass, mic, etc) in Garage Band using the TG voices, where I can then these convert to MP3, etc (that part I've got down). I also would like to be able to just play the keyboard and TG55 like I've always done, sometimes WITHOUT recording into the IRIG, but that's secondary to recording at this point.
Still confused re the audio cables also, I'll have to experiment some mare, sorry I'm so inept at this but thank for the nudge!