The next altar on the left-hand side is dedicated to St Sebastian, and the relic of his skull is venerated here. The altarpiece is by Giovanni Baglione depicting SS Irene and Lucina Tend the Wounds of St Sebastian. Below this is a diapered grating through which you can see the little Baroque shrine containing the skull. In between the grille and the altarpiece is an epigraph describing the enshrinement in 1632.

(Another photo of the altarpiece here)
(Another photo of the reliquary here)