Ineffable is basically anything that is unspeakable; unutterable; indescribable. The Ineffable plan is basically God’s will. Based on the proverb “God moves in mysterious ways,” this descriptive of God’s intentions is useful for agnostics who proclaim they don’t really know whether there is a God, and if there is, what on earth is he up to.
We as humble human beings will never know truly what the Ineffable Plan is whilst we shuffle our mortal coils. But if a heaven awaits us, then mayhap we will find out what all our sufferings were for. Until then, we can only ponder about the reasons why the world is like it is, and for everything unexplained by logic and reason, jot it up to the Ineffable Plan.