The correct order of increasing first ionisation enthalpy for Mg, Al, Si, P and S is

Across period 3 the general trend is upward, with two exceptions. Al ($3p^1$) is lower than Mg because the $3p$ electron is better shielded and higher in energy than the $3s^2$ pair. S is lower than P because $3p^4$ has paired-electron repulsion while P has the half-filled stable $3p^3$ configuration. Values (kJ/mol): Al 578, Mg 738, Si 787, S 1000, P 1012, giving Al < Mg < Si < S < P.