so basically, Al and S are slightly lower than the element before them - it helps to remember the overall trend on a graph of first ionisation energies across a period. Aluminium is starting to fill its 3p sub-shell ([Ne]3s23p1) and so has one electron in a 3p orbital. The 3p sub-shell is at higher energy than the 3s sub-shell so less energy will be required to remove the electron. With sulfur, electrons are starting to pair up ([Ne]3s23p4) - it already has an electron in each of its three 3p orbitals, so it's last electron has now paired up with another electron. This pairing will result in repulsion (since both electrons are negative), making it slightly easier to remove this electron and less energy is required.
Hopefully my explanation makes sense! It can help to look at orbital notations for the two elements (the boxes with the arrows representing electrons) just to visualise it too. : )