You wouldn't always get more TRS if you get married - it depends on how much interest you pay.
For example - I did not get anymore TRS when I married.
http://www.revenue.ie/en/tax/it/leaflets/it1.html#section7
Amount of Relief Available
2009: From
1 January 2009, First-time buyers - the rate of mortgage interest relief is increased from 20% to 25% in years 1 and 2 and to 22.5% in years 3, 4 and 5. The relief remains unchanged at 20% for years 6 and 7 of the mortgage. First time buyers relief ends after year 7.
Non-first time buyers - the rate of mortgage relief is reduced from 20% to 15%.
2010: Qualifying loans taken out before 1 July 2011 will continue to get relief for 7 years. Transitional measures will be provided for qualifying loans taken out between 1 July 2011 and the end of 2012.
Those whose entitlement to relief would, in the absence of this change, expire in 2010 or after, will continue to qualify for relief at the applicable rate up until the end of 2017.
The relief will be abolished completely by the end of 2017.
For more information see leaflet:
Tax Relief at Source (TRS) for Mortgage Interest Relief.
Relief available for loan interest on secured and unsecured loans for the tax years 2009 and 2010:
Personal Circumstances First Time Buyers All Others
Single €10,000 €3,000
Married/Widowed €20,000 €6,000
Note: Amounts shown in the above table are the ceiling amounts for the years 2009 and 2010