What is the trend in first ionisation energy across a period?

Overall, the general trend is an increase across the period.

The decrease in shielding (as outer electrons are less influenced by the nucleus) is outweighed by an increased nuclear charge and a decreased atomic radius.

However, the value actually decreases between group 2 and 3. This is a result of the electronic configurations of these elements. The outermost electron in Be (in a 2s orbital) is less easily removed than that of B (in a 2p orbital). The same occurs from group 5 and 6.