Fallon you can calculate whether this is the correct figure or not yourself. You know the way the bank sends you a summary of your mortgage and interest paid every year? You need to dig those out for the years 2007-2013 and look at each year individually. If you google 'TRS rates 2009' the first one on the list is a Revenue document, that gives you the rates for 2007, 2008 + 2009.
The rules were changed in 2009 so you can find the rates for 2010 + 2011 here: [broken link removed]
You can get the rates for 2012 and 2013 here:
http://www.revenue.ie/en/tax/it/reliefs/tax-relief-source-mortgage-rates.html
Basically you need to put together a table with all the years 2006-2013 as the first column. Then next column write out which year of your mortgage each of those years was so 2007 was yr2, 2008 was yr 3 etc.
Then you see how much actual interest was paid on your mortgage for each of those years (from the bank statements).
Then you see how much TRS you were entitled to for those years.
Assuming ye weren't married and ye were both first time buyers these would be the rates:
2007 (yr 2) you were each entitled to 20% (8000)
2008 (yr 3) you were each entitled to 20% (10000)
2009 (yr 4) you were each entitled to 22.5% (10000)
2010 (yr 5) you were each entitled to 22.5% (10000)
2011 (yr 6) you were each entitled to 20% (10000)
2012 (yr 7) you were each entitled to 30% (10000)
2013 (yr 8) you were each entitled to 30% (3000)
For all of those years, because your ex didn't deregister, ye were getting double TRS relief i.e. double the figures shown in my list here.
So if in 2007 the TRS you were given was 20% (8000) + 20%(8000) = 3200. But because you lived there on your own you were only entitled to 20% (8000) = 1600 so the refund you owe the Rev for that year is: 3200 - 1600 = 1600
However this is only the case
if the interest paid on your mortgage in 2007 exceeded 16,000 (8000+8000). As it was the first year of your mortage, it probably did.
The TRS you were given in 2008 was 20% (10000) + 20% (10000) = 4000. However you should have only got half that so you owe the Rev 2000 for that year. This is only the case if the total interest paid on your mortgage in 2008 exceeded 20,000 (10000+10000)
As the years go on, the interest on your morgage falls and so for e.g. 2012 the TRS you were given was 30%(10000) + 30%(10000) = 6000, but you would have only got that if the interest on your mortgage for 2012 exceeded 20000 (10000+10000) which it probably didn't. So the TRS you actually got was 30%(the actual interest) when what you should have got was 30%(10000).
So the refund for 2012 you owe the Rev will be 30%(the actual interest) - 30% (10000).
Do you get me? All you need is the interest figures for each year and using the rates above you can calculate what you owe the Rev.
From a quick glance, their figure of about 9grand looks about right to me. Considering the amount you would owe if ye had been given the full TRS entitlement every year would be 14,000 but it's obviously not that high because in the last few years the mortgage interest wouldn't have reached the double TRS ceilings. So 9 grand sounds about right but definitely check it out in a spreadsheet.
Also - you owe this money to the Rev, not your boyfriend. Basically since 2007 you've been paying a reduced mortgage based on claiming that he was still living in the house. He wasn't and therefore your mortgage should have gone up since 2007. So unfortunately it's you that owes this money to the Rev (assuming you've been paying the full mortgage since 2007 that is !!)