There are threshold reliefs available to you . As you are receiving from your Grandparent this will fall into threshold B you can receive roughly €47,000 from a grandparent since Dec 1991 cumulatively without incurring CAT. Anything above this figure you will be liable to CAT at 20%.
There is another relief available that if in the unfortunate circumstances that your grandparents child has pre deceased them (your parent) you can step into their shoes so to speak. you will then move into class threshold A giving you the ability to receive aprox €470,000 from them cumulatively since DEC 1991.