Why is ionization energy of indium less than gallium? In group 13 we observe an irregular trend in ionization energy: B > Tl > Ga > Al > In.

Gallium has a filled 3d subshell, but indium has a filled 4d and 3d subshell. Thus it should have more poor shielding along with more ionization energy, but that is not the case why?
The theoretical values of ionisation potential are pretty close to each other.in fact ionisation potential of gallium is almost equal to that of aluminium.Now due to MORE shielding in indium but lack of lanthanoid contraction the affect balances out.As the atomic radii and shielding affect simultaneously takes place from your viewpoint you can claim that the increase in the atomic radii predominates over the zeff.thus indium will have lesser ionisation energy.