I've done some mad googling and just can't seem to get a definitive answer that I can be certain about.
I'm registered to vote in Galway East. I want to change this so I can vote in Dublin Bay South.
I think, though I could well be wrong, that this is the form I should use: RFA3 Change of Address. But is that right - is it only a supplement to the register that I'm making? And do I post it to Galway County Council?
Yeah, the register is set on the 15th of Feb each year, any changes after that are supplemental. You want to fill out the RFA3 and send it to Galway in your case, they will remove you from their register, and forward it on to your new Local Authority.
And then will that be permanently changed - as in, it'll be formally part of the register come 15th of Feb?
It's updated on an annual basis. Draft register is published by each Local Authority at the start of November. This may be done by house-to-house visits, or just a form posted out to each household for completion and return.
Because you will be put on Dun Laoghaire - Rathdown's supplementary register, I don't believe (but open to correction) you'll automatically be added to the draft register in November (unless there are any elections called), which then becomes the Live Register in Feb after any corrections. To do that you want to use the RFA Household form, or because you are in Dublin you might be able to do it through voter.ie if you have a myGovID