The marker for irrealis may be a verbal auxiliary or it may be best analyzed as an adverb (consider its position to the left of the subject in the example given). More research on its use in discourse is needed.
I can't find examples of IRR + ANT + PROG/HABIT in one clause. As Examples 76 and 77 show, IRR co-occurs with quasi-modals like go + main verb, and can co-occur with stap (PROG/HABIT). Bin (ANT) co-occurs with stap. Putting them together (e.g. in a counter-factual such as If he had been working there, he would have been paying you) is very awkward (maybe ungrammatical).
Source: Meyerhoff nd