You don't say if you're married or single or over or under 55. If you are single and under 55 you are entitled to a maximium tax relief at 20% of €1800 = €360 regardless of what marginal tax rate you're on. Based on the about of rent you pay you're are entitled to the maximum tax relief i.e. €360.
You need to complete a Form Rent1 available from the following link: [broken link removed]