The arrival of Christianity marked a decline in the old order and the dawn of a new era. Many of the existing religious and ceremonial sites of the Druids were Christianised and adapted by the new religion.

The founding of the first church by Saint Fiachna, the patron saint of the parish, at Droum-Fiachna, in the town land of Garranes is a case in point. The unique Bullaún stone nearby, known as the Rolls of Butter, while closely associated in legend with the saint, is believed to have been used by the Druids as a ceremonial site. The present ruins in Saint Fiachna’s Cemetery, which may well have replaced an earlier church, or churches, dates from the post-Norman period.

The remains of three other churches from this period are to be found in Bonane. The earliest, Faill a Shéipéil, at Gearhabuí is now a mere outline. The next, Sheana-Shéipéil, the ruins of which are still clearly visible near Bonane Bridge, was in use until about 1840. It was described in 1839, by the then Parish Priest, Michael Enright as one “with tottering walls…wretchedly confined unsafe cabin”.

Shortly thereafter a new church was constructed at Milleens, on the site where the present church stands. One wall of this church is still well preserved. It was a relatively low structure with the Sanctuary at the northern end, which is opposite to the arrangement in the present day Saint Fiachna’s Church

The present church dates from 1892 and is built of local stone. The nearby Presbytery was completed two years later. Prior to its construction the priests of the parish resided at Releagh. The residence there was built for the engineer in charge of the construction of the New Line road through the Tunnels to Glengarriff.