I have been an Electric Ireland customer for a number of years. I recently moved address but like in my previous address, I am the account holder. I missed a payment and got a letter saying after a review of my account a €300 deposit is required immediately. 
I went to pay the bill and it took for the full amount of €450 (including this "deposit"). 
I now am low in funds and want to know on what grounds this was added out of the blue and if it will be refunded.

The €300 was probably a computer generated bill. I got one when I moved and I was on direct debit. However a short phone call later and I was told that it would be cancelled; which it was.

Hi there,

We will look into this ASAP - just private message me the Electric Ireland account number, name, address, telephone number on the account, and state in the message (for GDPR/Data Protection) that you are the account-holder.

Thanks,
Una

Thank you, I rang up and it was refunded. I'll call off the cavalry :-) 

Hi there,

Thank you for letting us know!

Kind regards,
Una