Here's a brief take on what I think the themes represent, please let me know if you agree

The cutting of the fingers I thought was the most obvious analogy once you're aware of the irish civil war theme.

During the Irish civil war, many people who had fought on the same side for years ended up fighting against each other. (If you want to watch a good movie on this theme alone, I recommend "the wind that shakes the barley", directed by Ken loach starring Cillian Murphy")

In banshees' ,Gleeson's character represents Ireland and it's traditional culture, not only does he play music, he creates original Irish scores and teaches the next generation of students.

Farrell's character represents those wanting to break away and stay within the British empire. Gleeson cuts his fingers off, a cryptic, horrific, but powerful message that he can't be friends with Farrell anymore, because it will result in him losing his cultural identity.

If there's any doubt that Farrell represents Britain, or at least the people who wanted to stay loyal to Britain, he confirms it towards the end of the movie by firstly invading, and later burning down Gleeson's home.

In the last few scenes Farrell and Gleeson stand side by side on the beach, separated by the Atlantic ocean, gleeson on the left and Farrell on the right, respresing Ireland and Englands positions as if looking on a map.

Other than the civil war theme I felt the movie had a lot of other underlying themes that stand out throughout the movie.

Mcdonangh had recently been through a bad breakup while writing the movie and this IS, essentially a break up, Farrell falling victim to gleesons sudden 'ick' but progressively turning from victim to villain as he refuses his ex partners wishes to be left alone.

Other themes that really stood out were parenthood and death. In what I thought was one of the most beautiful scenes, Gleeson picks Farrell up after being punched by the cop. He starts to bring him home on the cart and is sympathetic when Farrell starts to cry like a father to a son. He pats him on the hand but then hands him the reigns and departs, him ascending up the hill and leaving Farrell to go out opposite direction at the crossroads.

In another scene, Siobhan is beckoned by the banshee on the opposite side of a lake. She refuses to cross over but Barry keoghan enters the frame and we can see that maybe it wasn't her she was signalling.

Siobhan eventually leaves the island, the banshee watching down on her from a cliff as she sails away. She is standing to the right of Farrell, the two separated by the Atlantic ocean, her a symbol of death, death of Irish culture and Farrells character too blinded with sadness and rage to notice her.