How would you account for the irregular variation of ionisation enthalpies (first and second) in the first series of the transition elements?

Ionization enthalpies are found to increase in the given series due to a continuous filling of the inner $d$ -orbitals. The irreqular variations of ionization enthalpies can be attributed to the extra stability of configurations such as $d^{0}, d^{5}, d^{10}$. since these states are exceptionally stable, their ionization enthalpies are very high.

In case of first ionization energy, $Cr$ has low ionization energy. This is because after losing one electron, it attains the stable configuration $\left(3 d^{5}\right) .$ On the other hand, $Z n$ has exceptionally high first ionization energy as an electron has to be removed from stable and fully-filled orbitals $\left(3 d^{10} 4 s^{2}\right)$

Second ionization energies are higher than the first since it becomes difficult to remove an electron when an electron has already been taken out. Also, elements like $Cr$ and $Cu$ have exceptionally high second ionization energies as after losing the first electron, they have attained the stable configuration $( Cr ^{+}: 3 d^{5} $ and $ Cu ^{+}: 3 d^{10})$. Hence, taking out one electron more from this stable configuration will require a lot of energy.