Why is the first ionization energy of boron lower than beryllium?

The rise in ionisation energy is not regular. To explain this we must remember that the second shell of electrons is actually subdivided into 2s and 2p. The most easily removed electron in Boron is that in the 2p orbital, higher in energy than the 2s electrons in Beryllium. It therefore needs less energy for total removal than does the 2s electron of Boron. This outweighs the effect of the increased nuclear charge of Boron, which tends to make its outer electron harder to remove.