Suppose my ally has been hit with a banishment spell and sent to another plane. I would like to bring them back by casting dispel magic to end the banishment spell. However, I immediately run into a problem: I need to "choose one creature, object, or magical effect within range" that is affected by the spell, and it's not clear whether I can do that. I can't target my ally directly because they are on another plane. Is there still a "magical effect" within range that I can target (perhaps some sort of "anchor" that marks where the creature will return to when the spell ends), or is it impossible to dispel the banishment because the only affected target is on another plane?
(Note: The question title says "effectively immune" because clearly banishment is not literally impossible to dispel. If I could get to the plane where the target was banished to, I could dispel the spell, but that is unlikely to be practical.)
Best Answer
Your Interpretation is Correct
According to Jeremy Crawford:
Of course, since Banishment is a concentration spell, there are options that you could take to attempt to end the spell if you are on the same plane as the spellcaster but not as the target (like casting Magic Missile at the person concentrating on it). And like you said, if you could somehow end up on the same plane as the banished creature, you could dispel the Banishment spell there. But yes, specifically the Dispel Magic spell wouldn't work unless cast upon the target of the Banishment spell.