The banshees of inisherin is about emptiness.
Boredom, futility, despair. The priest even asks about it.
Nothing happens. Absolutely nothing. They are so alone on that island, detached from the entire world. Even war doesn’t reach them. They don’t have any experiences, any impressions, any hopes or strivings or dreams.
Colum is terrified of it. The futility of his life, the fact that he will die without leaving any trace on this earth scares him more than he can comprehend. But even in the present moment it is impossible to live with.
People need experiences and communication to live through things happening inside. You process grief and joy and terror through things happening. It’s very hard work to process them yourself. And there is absolutely nothing happening on Inisherin, nothing for any of them to live through. So they create occasions, otherwise they might as well go mad.
Colum cuts his fingers off because then he can’t play anymore that way. Not leaving a trace on this earth isn’t his fault now, how can you play the violin without five fingers? He doesn’t stop Podric from burning the house down.
And after it all happens? After he’s lost half his fingers, after his home is lost to the fire, after his friend’s donkey chokes on, again, his own fingers, after a boy is found dead in the lake, after all of that he starts talking to Podric again. Because he’s lived through the terror of the futility of life. He is free of its weight on him, and he used Podric to do it.
So the banshees of inisherin is about emptiness. Not having anything to go through. Not having anything to fight against or for, to love, to hate, to destroy, to enjoy. There is a reason why one of the worst sentences that can be brought out is solitary confinement.