"The shop was being closed."
Translation:Bhí an siopa á dhúnadh.
How could this be said using the "ag" form instead of this passive "á" form?
What do you mean? This is basically Tá an siopa ag/do a dhúnadh - same as the pronomial form (tá sé do/ag mo bhualadh). But ag/do a here becomes á.
Okay I didn't know that, thanks. Now I guess I just don't understand why it's "ag a dhúnadh" rather than "ag dúnadh". Is it because the shop must be "at its closing" and can't actually DO the closing?
It's just the way to form the passive. I don't know the history of the construction.