Isaac Asimov answered this question rather neatly, I thought. I can't remember in which of his many books I read it (it was a long time ago), but the gist of his argument was this: A universe in which there exists such a thing as an irresistible force is, by definition, a universe which cannot also contain an immovable object. And a universe which contains an immovable object cannot, by definition, also contain an irresistible force. So the question is essentially meaningless: either the force is irresistible or the object is immovable, but not both.

This was my first introduction to philosophy. It was also my first introduction to the notion that ideas which are actually incoherent, when analysed, can nevertheless be extremely useful metaphors. I can think of no better way to describe some encounters between two-year-olds and their mothers, for example.