Similar thing happened me, received a fine as a rubbish bag was dumped illegally. Litter warden found a letter with my name and an address, which I hadn't lived in for 2 years. They got my correct address and I received a fine by registered post.
I rang the litter warden and was told I could go before the courts and explain my case, but it would probably be cheaper just to pay the fine, I can find better things to do with €150.
I wrote a letter to the county council, explaining I had not lived at that address for two years,attaching proof of my new address and also some bin tag receipts (to show I had no need for illegal dumping). I also gave the contact details of the landlord to verify I no longer lived there.
They replied that they were satisfied with my explanation and the fine was dropped. Check if there is any appeals procedure, if not, write a letter stating your case.