The same thing happened to me last month (my TRS went down by €58!) and it turned out that it was because TRS goes down in year three of the mortgage. I knew that, but didn't realise that Revenue calculates by tax year, so even though my mortgage started in October 2009, October to December counts as year 1, and 2010 was year 2, so January 2011 is year three and that's all there is to it. Get in contact with Revenue and they should be able to clarify.