Here it is
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First ionization energy depends on  the size of the atom and the stability of electronic configuration

Down the group size of the atom is changing but electronic configuration remains the same so generally down the group the trend is regular and ionization energy is decreasing due to increase in size
However across the period electronic configuration is changing and in between we are getting very stable fully filled or exactly half filled electronic configuration This results in sudden increase in the ionization energy. For example in barium family and in in nitrogen family.
Beryllium has more ionization energy than boron due to fully filled electronic configuration
Nitrogen has more ionization energy than oxygen due to is extra stable exactly half filled electronic configuration

In transition elements the d subshells are not field monotonically with increase in atomic number
Exceptions are there where electrons are filled  to acquire exactly half filled of fully filled electronic configuration examples chromium and copper families