San Barnaba de Porta was a small 18th century (?) chapel just inside the Porta Maggiore, in what is now the Piazza di Porta Maggiore in the rione Esquilino. It was demolished in the 19th century.
"De Alvariis" has flagged this obscure chapel in his massive collection of photos of Roman buildings in Flickr online. It appears in two engravings that he reproduces:
The first is by Piranesi, dated 1784. This shows a small rectangular edifice, with a lower lean-to annexe on the left. The façade has a single doorway flanked by two vertical rectangular windows with Baroque frames, which occupy the entire width of the frontage on each side. Above the door is a cartouche of the same width as the doorcase, and in the gable is an oculus or tondo (it is not possible to discern which). On the point of the gable is a gabled bellcote containing a single bell and having a cross finial. On stylistic grounds the edifice can be dated to the first half of the 18th century.
The second is by Luigi Rossini, dated 1825, and this shows the same building from the side.
De Alvariis labels this church as Santa Barbara, but also suggests that it may have been identical to a San Barnaba de Porte which is listed by Armellini in 1891 as follows:
San Barnaba. De questa tacciono tutti i cataloghi più antichi, tranne quello del secolo XIV di Torino. Da questo apprendiamo che stava presso la porta Maggiore e perciò era detta S. Barnaba de Porta; era servita da un solo prete.
(There is similarity between the names Barnaba and Barbara in Italian.)
Before the 19th century, the Porta Maggiore was isolated in open country. The engravings show a terrace of two three-storey houses on the left hand side of the gate heading outwards, and a farmstead opposite on the right hand side. Back then, there was only a narrow archway open for the road. The chapel was attached to the end of the terrace. Presumably it served the people living here, as well as the guards manning the gate.
The Nolli map of 1748 shows a vacant plot where the chapel should be, and if this is correct it was built soon after.
Pope Gregory XVI ordered the gate to be cleared of accretions in 1838, and the demolition of the chapel would have been expected then. However, Rossini published an engraving in 1850 which shows the chapel still standing, but without its bellcote which hints that it was deconsecrated. It was certainly gone before 1870.
The location is marked by the tramway on the other side of the grassed area of the gate on the city side, just north of the triangle formed by the tracks.