It will depend on your salary and what your top rate of tax is. If you have paid tax at the 20% rate only then you will get 20% relief. If you paid tax at 41% then you will get 41% relief, depending on how much salary was taxed at 41%.
Eg - if, say, your medical expenses came to €2,500 and you paid tax at 41% on €1,000 of your salary, then you would get tax relief at 41% on €1,000 of the medical expenses and 20% on the balance.
Medical expenses are one of the few reliefs that are still available at the high (41%) rate.
If you paid no tax then you will get no relief.