So, what is the difference between the "past independent" (of which this is apparently the form of the verb déan) and the "past dependent"? I know it shows up in other tenses too (is and tá for the present copula, I guess), but in the past, and with a verb like déan, it's new to me.
Actually, is and tá are different verbs. tá is a form of the verb bí ad has a dependent form fuil, seen in an bhfuil and níl. Faigh has one in the future tense.
Basically, one is used by itself, the other is used with particles. However, some particles use the independent form.
Dependent forms typically follow verbal particles such as an, ar, ní, níor, etc.