Most likely, the forces generated would punch a hole in the fabric of reality and you'd get your answer just as you blinked out of existance. Or it would become the fabled fountain of kittens.
Either way I win.
They collide and the fabled fountain of kittens bursts forth from the ground, spewing awesomeness and cute in every direction, and before long you will begin to hear a yipping sound as the puppies make their way out of the fountain as well!
What I think would happen is that the force would meet the object, and since this force is unstoppable, not unslowdownable or unchangedirectionable, it would find it's way around the object by exerting force in many directions and following the path of least resistance.
Well, logically speaking (or as logical as this BS hypothetical situation can get) the only way a force could be irresistible is if it's being propelled by an infinite amount of energy. The only way an object could be immovable is if it has an infinite amount of mass. Something moving at the speed of light crashing into something with infinite mass would create and explosion that would make the Big Bang look like a fire cracker.
Of course, infinites don't actually exist.
Logically speaking, the irresistable force would just move everything around said object.
Space is a funny place.