He didn't really seem to add anything in the movie for me. I mean Fergus hits to get him to back off of Dil and gets him off her back, and then gets Fergus some points from Dil, which is what the story needed.
However, even if Dave is used for that, he keeps on coming back after and I didn't really see the point. Why not just have Fergus and Dil develop their relationship without some other competitor, getting in the way?
I wondered why he was in the movie also. They only thing I can think is when the car was 'terrorizing' Dil and Jimmy in front of Dil's apartment she says 'Go home Dave' and 'I've never seen him act like that before.' Maybe they needed a crazy character so people wouldn't think it was the IRA members after Jimmy/Fergus but Dave. The irony is they never showed Dave driving a car. He was always on foot so how could he have suddenly been driving around in a car? That was strange.
sharecomic relief
shareHe adds to the ruse of Dil being a woman - an abusive lover who calls her ‘bitch’ and ‘cunt’, and from whom Fergus can save Dil, further entrenching the stereotype of the helpless female.
Dave also acts a red herring - they assume he’s driving the mysterious car that follows and nearly hits them, when actually Fergus’ IRA pals are back.
But yeah, he’s mainly there to cement the sausage twist.