Had similar problem. Contacted o2 in the end who said they could ban the number if I got a letter from the gardai.
Went to the station feeling very foolish (it was my last resort after nearly three years of phonecalls. Sometimes not for months and then they'd start again. I didn't want to go to the gardai and didn't want to get the person into any sort of trouble but was getting calls throughout the night and just wanted it to stop). The garda was really nice and contacted o2 who then said it wasn't possible to ban an incoming number. I got the impression it was more to do with where the calls were being made from rather than the number.
Gardai made one phonecall. Nothing serious, not even a warning. I think it was just enough for the caller to know I had actually gone to the gardai. Got two texts the same day as the gardai called and nothing else to date.
If its really annoying you then this route could be worth looking into.