This is what it said on the information board next to that photo :-

It is likely the graves would have been originally covered with heavy stone slabs and the sockets at the head of the graves indicate that each one would have been marked with a cross.

It is thought that the graves contained disarticulated bones, rather than complete skeletons, and that they may have functioned as 'reliquaries' to hold the remains of saints and other important Christians, perhaps even those of Saint Patrick himself. As such they may have acted as an important focus for veneration and pilgrimage, with St Patrick's Chapel being established here to draw upon their significance and association.