Along a period the shielding remains the same. With the increase in the number of protons acting on the outermost electrons, the radius becomes smaller and therefore there is an increase in the ionisation energies.
This patter is true apart from when either the s orbital is filled up or the p orbital is half full. In any of these two cases, there is added stability, resulting in a bit more energy being needed to remove an electron. This results in a drop in energy needed to lose an electron for Grp 3 and Grp 6.
B to Be
Be has a full 2s orbital whilst B has an electron in the 2p orbital. This full 2s orbital offers some extra stability, making it easier for B to lose an electron, and thus a lowering of ionisation energy.
N to O
N has a half full 2p orbital whilst O has electrons paired up in the 2p orbital. This half full 2p orbital offers some extra stability, making it easier for B to lose an electron, and thus a lowering of ionisation energy.