The given English translation seems a little awkward to me (American Mid-Atlantic English), but this course seems to value literal and structure-preserving translations highly.
In what circumstances might this sentence be said? Could I say this while pointing to a picture I took during a road trip? Could a literary character say this as an apostraphe or soliloquy to the audience? Could I say this in response to the question "what are you doing here?"
Another way of asking this question is, could I rephrase this sentence in English to any of the following: "I am on a journey," "This is my journey," or "I am the one on a journey"?
I agree. Some alternative meanings:
I am here right now because I am on a journey and it has taken me to this place at this time, so "I am on a journey here". Táim ar aistear anseo.
You are looking at a photograph of me taken when I was on a journey, so with the scene in the photo in mind "I am on a journey here". Táim ar aistear anseo.