On a hilly ridge between the Calore and Ufita valleys, Mirabella takes its second name from the ancient Aeclanum, whose site is in the Passo di Mirabella hamlet.

The excavations of the last century have brought to light important evidence of the Samnite settlement, then expanded and transformed by the Romans, today an archaeological park open to the public. Oscan inscriptions found on the site have made it possible to date it to the 4th-3rd century BC. the foundation of the important city of the Irpinia, on the front line during the Samnite Wars. Passing under Roman control, Aeclanum, in a strategic position on the trade routes crossed as it was by the Via Appia and close to the Via Aemilia Aeclanensis and the Via Aurelia Aeclanensis, grew in extension, protected by an imposing wall that reached ten meters, with towers and three access doors. Thus, the Appian Way entered the city through the western gate and left through the eastern one. This long stretch of the regina viarum is among the major attractions of the archaeological park, together with the well-preserved thermal baths of the imperial age, the ancient macellum, or the market square, the shops, the remains of houses and a domus from the 1st century AD., to the early Christian basilicadating from the Justinian period with a mosaic floor and a baptistery. The finds found in the excavations are in the Irpino Museum in Avellino. Even older, in the hamlet of Santa Maria delle Grazie, is the archaeological site of the Eneolithic necropolis of the Gaudo culture, dating back to the 3rd millennium BC, with tombs from the underground burial chamber.