You could swap your coins for notes with someone who is travelling to the UK/NI and has bought sterling - they might be glad of the change, and you can then exchange the sterling notes for euro.
I presume it's because there is no demand here for Sterling coins, so they would end up having to send them up north which would incur a significant cost.
Provided the banks will take them from non-account holders. My experience in changing my "piggybank" is that AIB and BofI will only handle coin for account holders. Halifax didn't take coin bags either, although I was an account holder.
Mrs Shark has informed me that Dunnes Stores in Newry will exchange sterling coins for notes with no handling charge. The manager told her he was "delighted to be getting them".