Hi,
I have been through the VRT appeal process about 2 years ago - it takes a while - as far as I remember it takes a couple of months to get an initial acknowledgement of the appeal in writing, and then another few months to get a result - a cheque hopefully.
In my case I was refunded about 25% of the payment - approximately reflecting the valuation I argued (with supporting documentary evidence) would be more accurate initially.
There was an aritcle in the Irish Times a week or 2 ago about appeals - most are successful apparently, but very few people actually bother to appeal.
Good luck.