why ionisation enthalpies along 3d transition series show irregular trend?

The irregular trend in 3d series is due to the fact that the removal of electrons alters the relative energies of 4s and 3d orbitals.

Thus there is a reorganisation of energy accompanying ionisation which results into the release of exchange energy which increases as the number of electrons increases in the dⁿ configuration and also from the transference of s-electrons into d-orbitals.

 

Therefore,  Chromium has low first ionization enthalpy because loss of one electron gives stable configuration (3d⁵), while Zinc has very high ionization enthalpy because electron has to be removed from 4s orbitals of of stable configuration (3d¹⁰ 4s²)