If an immovable object is an object that cannot be moved no matter what and an irresistible force can move anything, what would happen if an irresistible force is used on the immovable object?
This is an unanswerable question due to contradictory premises.
The immovable object vs. the irresistible force question is a commonly used example for this fallacy of reason, and it is explained on this page:
The problem here is that in a universe where an irresistible force has been defined to exist, there cannot also exist an immovable object, because then the force would not be irresistible. Conversely, if there is discovered or defined such an item as an immovable object, then by definition there can be no such thing as an irresistible force.
It's a fallacy of reason only if you "assume" that the irresistible force and the immovable object are two separate entities. If they are one and the same, then the statement "they can't meet" holds true. Now, if the universe is the irresistible force and the immovable object, then the logic holds.