If it is an inheritance - ie someone left it to you in their will and they have died - there is no capital gains tax. There may be Capital Acquisitions Tax (sometimes called Inheritance Tax).
If it is a gift, then the person transferring the farm may have a liability to Capital Gains Tax and you may have Capital Acquisitions Tax.
Capital Acquisitions Tax is charged at 20% once the taxable value of gifts and inheritances go over a certain amount. That amount, for gifts and inheritances from parents is €521,121. However, you might qualify for Agricultural Relief. If, after the inheritance, the total value of your agricultural assets (eg farm, livestock, farm machinery and farm house) makes up at least 80% of your assets, then for the purposes of CAT, the inheritance of the farm will be treated as if it is only worth 10% of its current market value.
In any case, you need to talk to your accountant.