(A) : In IIIA group ionization enthalpy do not decrease smoothly down the group. 
(R) : In the heavier elements of IIIA group the ‘d’ and ‘f ’ electrons have poor shielding effect.

I.E. Order: B >Tl >Ga > Al > In

Atomic radius order: Tl$\approx$In >Al > Ga > B

                           OR

                         B < Ga < Al < In ~ Tl

 Ionization energy order of IIIA group elements is, B >Tl >Ga > Al > In

On moving down the group, the ionization energy decreases, this is true for B and Al. The ionization energy of Ga is unexpectedly higher than that of Al.

This is due to poor shielding effect by completely filled d-orbitals in the inner shell of Ga, hence the Ga valency electrons are strongly held by nucleus therefore removal of electron from Ga is highly difficult compared to Al.

A similar increase is observed from In to Tl due to presence of 14 f-orbital electrons in the inner shell of Tl which have poor shielding effect.