Thanks Carmanach that’s extremely helpful.
So originally we had:
Dá bhfeicfinn é, labharfainn leis = If I see him, I would speak to him.
Dá bhfeicinn é, labhras leis = If I had seen him, I would have spoken to him.
So for closed conditions:
Past subjunctive in the subordinating clause, past indicative in the main clause.
For open conditions:
Conditional in the subordinating clause, Conditional in the main clause.
However the open condition syntax is now often used in both cases.
This is interesting, because Peadar Ua Laoghaire and other Cork writers, such
as Dónall Bán Ó Céileachair, often display a mixed form:
Dá bhfeicinn é, labharfainn leis
With past subjunctive in the subordinating clause and the conditional is the main clause.
This is used for both open and closed conditions.
By the way Gnás na Gaeilge is a fantastic book, the explanations of how you can use either
verbal noun structures or “particle + verb” in temporal, spatial, e.t.c. clauses is quite detailed
and not really explained in any other book that I have read.