The old church of Saint Mammens was located close to or on top of the ruins of the Roman theatre, which means that, if the inscription was near its original location, then it could have been standing in a public area around the theatre or by the road that led to the river bank, a part of which has been excavated in the cloister of the medieval cathedral. As far as I know, there are no traces or even hints of a temple to Concordia.
There are news of an inscription to Concordia either next to or in the old church to Saint Mammes, which was destroyed in 1755 earthquake (but memory of it survives in the street name Rua de São Mamede). The reference appears to come from 18th century’s or earlier documents, but the votive monument itself didn’t survive. According to Vieira da Silva, it read: CONCORDIAE SACRVM M. BAEBIVS M. F. M. M. FELIC. IVL. DAT. (1944, 118; N. 24).