The Christian Brothers say that when an indefinite noun that is qualified in some way follows on a verbal noun, the indefinite noun (obair)actually stays in the nominative
= ag déanamh obair mhaith.

whereas if the indefinite noun was not qualified, it would stay in genitive as is the rule following verbal nouns
= ag déanamh oibre.

I'm okay with those rules re. verbal nouns, but then I assumed this same rule would apply to the compound prepositions also. But instead I am seeing the genitive used even when the indefinite noun is qualified. Here are two examples I came across lately
= in araíocht ruda eile
= go ceann scaithimh fhada

Is this the case? I really expected to see 'ruda' and 'scaithimh' in their nominative forms 'rud' and 'scaitheamh'

Yes it is, barring any action in the multiple genitives rule. To tell you the truth, I have no idea historically why the nominative form comes into play when there is a indefinite noun with an attributive adjective. Maybe it was a separate form which melded with the nominative over time, and everyone forgot the roots. But the two processes are separate.

Thanks Sean, glad to know this for sure as it comes up a good bit.