'Of the foe's charge, or when defending, one may place his Cliarthairi forward, and the warbands behind, or upon flanks, and mutually then they pelt the foe with missile, and if horse there is, they may countercharge from the left or right flank, with missiles on the opposing. The soldiers fore hold close their shields and absorb the charge, then press forward and push the enemy back, as the warbands or horse encircle the flank'
A few things;
here the 'heavy infantry' are in the front, in a shield wall. On the left or right flank are missiles, light infantry behind in the center, and the opposing flank has the cavalry.
'Three lines then of the men (the Cliarthairi; heavy/elite infantry, the regular units), with the warbands before them (missiles/skirmishers in front) and the footmen on their sides (light infantry on flanks), and horse behind'
Essentially, heavy infantry in three lines in front of one another; missiles in front of them, light infantry to the flanks, cavalry behind the formation.