Re: ESB Bill
It's bad form on behalf of the landlord that he didnt' do this. Normally when a tenant moves out, the landlord (or his agent) will check the apt and make sure that all is OK. If all is OK, the deposit is returned to the vacating tenant. A reading should then be taken of the ESB meter and the tenant should call the ESB and get the bill transferred to the landlord's name straight away. I know the landlord wasn't fortcoming with the whereabouts of the meter, but the onus is still on the bill holder to get this sorted. You should have called the ESB at that stage and explain the position to them.
If you were in the apartment for some time, the ESB should be able to provide an average monthly usage for you.
To me it looks like the people who moved into the apartment after you are getting free electricity. They're not paying the bill because it's still in your girlfriend's name. You should call around to the apt and tell the tenant's who you are and ask them if there's any "post" there for you. If there is, look for the ESB bill that was issues after you moved out. You could work out a daily cost and pay this amount. It might cost you a few € more to do it this way, but at least you'd have paid for what you used.