IP₁ is least for

Atomic radius order :Tl~In >Al > Ga > B

                           OR

                         B < Ga < Al < In ~ Tl

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

On moving down the group ,the ionisation energy decreases,this is true for B and Al.The ionisation 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 .