In general from left to right in a period, ionisation enthalpy increases due to effective nuclear charge increases. but due to extra stability of half filled and full filled electronic configuration, required ionisation enthalpy is more from neighbouring elements.
i.e. first ionisation enthalpy order is

$\mathrm{Al}<\mathrm{Mg}<\mathrm{Si}<\mathrm{S}<\mathrm{P}$