Why does first ionisation energy decrease as you go down a Group in the Periodic Table?

There are two reasons: increasing atomic size and increasing shielding effect.
Increasing atomic size - As you go down the Group, the atom size becomes larger, therefore the electron that must be removed is further away from the nucleus. This increasing distance decreases the attraction of the positive nucleus for that electron, therefore less energy is needed to overcome this attraction and remove the electron.Increasing shielding effect - electrons in the inner energy levels will shield those in the outer energy level from the full nuclear charge and so will decrease the attraction between the positive nucleus and the electron to be removed. Therefore, it will be easier to remove an outer electron.