“Properly speaking “necessary” and “contingent” are consequent upon being, as such. Hence the mode both of necessity and of contingency falls under the foresight of God, who provides universally for all being.” Aquinas, Summa Theologica
This post will be short.
When thinking about God’s created effects, on the doctrine of omnicausality (given by classical theists) God as the first and timeless cause creates not only a particular effect, but even the modality of the effect. Hence when God creates something that is in potential – say a seed that could possibly become a tree – he creates not only the seed itself but even the very possibility that inheres in its being able to become a tree. What this means is that possibility inheres in an object at a particular time. That is, even if the seed never in fact becomes a tree, it could still be the case that at time 1 it stood in a potential relation to actually becoming a tree. What I’m pointing out is that it is not necessary for a thing to actually become what it is in potentiality of becoming in order for that thing to actually be in potentiality towards whatever it could become. I.e. a trillion acorns could all never in fact become trees (say the earth is incinerated by a comet), and yet it could still be the case that, prior to the comet, every single one of those acorns had within itself a real potential to become a tree.
Imagine then all moments of time, spread out on a line. Now imagine God creating, at each moment, not only every particular effect at each moment of time, but also each modality that inheres in each effect. This would include free will acts themselves. Not only their existence in act at each particular moment, but also their potentiality to be in other acts as well.
Alfred Freddosso has a great article that sort of goes into this “Dominican” position here: http://www3.nd.edu/~afreddos/papers/freedom%20and%20God.pdf
Now, if this view is right, and that the very modalities of “contingent” and “necessary” as they relate to free willed acts themselves are things created by God, then perhaps we can get a better idea about how to understand God’s irresistible and resistible graces.
Irresistible grace is just such a grace that cannot be resisted, for that “cannot” is the very modality that God has created in the grace itself. Resistible grace, on the other hand, “can” be resisted, and in fact will be, if God does not supply additional irresistible grace, for that is the very modality that God has brought about when creating resistible grace; i.e. whether it actually moves from a “can” to a “will” depends on further divine acts of God.
Much more could be said about that last paragraph. My point in this post is simply that something can still be “possible” even if God has ordained that in fact said possibility will never be actualized.