Iria Flavia or simply Iria in Galicia, northwestern Spain, was a Celtiberian port, the main seat of the Caporos, on the road between Braga and Astorga. The Romans rebuilt the road as via XVIII or Via Nova  and refounded the Celtiberian port as Iria Flavia ("Flavian Iria") to complement Vespasian. Iria was the seat of a sixth-century Christian bishopric that shared its seat with Compostela, then moved there in 1095. The modern city on the site of Iria Flava is Padrón.
The followers of Priscillian were deeply embedded in the culture of Iberia's northwest. To restore catholic orthodoxy in the Visigothic marches that were recovered from the Kingdom of the Suevi in a series of campaigns during the years leading up to 585, nine dioceses were established in Galicia, including Iria Flavia, mentioned in the document Parroquial suevo (ca 572–582); the Parroquial divides the region into dioceses and marks the first definitive integration of this zone in the monarchy of the Visigoths, who had been catholicized from Arianism in 587 (Quiroga and Lovell 1999). The list of the bishops of Iria present at councils and noted in other sources begins in the sixth century with an Andreas and gains historic credibility in the seventh . No commercial or political rationale for siting a bishop at Iria Flavia seems to present itself, though excavations have identified a cult sanctuary dating to the second half of the sixth century (Quiroga and Lovelle 1999). The relics that were identified with Saint James the Greater and which were transferred to Compostela may originally have determined the location of the diocese at Iria, to control the already sanctified site.
At any rate, otherwise unidentified considerations dictated that the new bishopric take the place of the older bishopric at Aquae Celenae, which was a Roman municipium and administrative center that was formerly of considerably more importance than isolated Iria.
Under Adaulfus II the city was destroyed by Norse pirates, and bishop and chapter took refuge behind the strong walls of Compostela. Soon they petitioned Ordoño II of León and Pope Nicholas I to permit them (c. 860) to transfer the see from Iria to Compostela, near the sepulchre and church of St James (founded c. 835). Both pope and king consented on condition, however, that the honour of the see should be divided between the two places. From the second half of the ninth century the bishops of this see are known as Irienses or Sancti Jacobi, even ecclesiae apostolicae sancti Jacobi—though no apostolic succession was possible—and finally as Compostellani (Catholic Encyclopedia). In 1095, through reverence for the body and the sepulchre of St James, Urban II, by a Bull of December 5, withdrew from Iria its episcopal rank and transferred the see in its entirety to Compostela, in favour of the Cluniac bishop, Dalmatius, present at the Council of Clermont that year. At the same time Urban exempted it from the authority of the metropolitan and made it immediately subject to the Holy See.
About the year 1000 Diego Gelmírez, bishop of Compostela, rebuilt the former cathedral church, Santa Maria Adina, which had been destroyed by Almanzor. Excavations have revealed that the site was built on Roman foundations. A Roman votive figure of a bull has been found, published in Corpus Artis Gallaeciae .
As the legend of Saint James the Greater having proselytized in Hispania spread, Iria Flavia came to be accounted the first site of his preaching.