The vault — believed to date from the 1700s — was discovered in the 900-year-old St Mary’s Collegiate Church in Youghal.
During excavations, the archaeologists also found evidence of centuries-old heating systems.
The vault, 30cm beneath the surface, was unearthed by Daniel Noonan, who runs an archaeological consultancy agency, working with John Kelly of David Kelly Partnership.
They were investigating the floor’s subsidence in a €60,000 restoration project funded by the Heritage Council of Ireland. The stone vault was crisscrossed by protective pine beams between it and the floor.
There was reportedly no obvious indication as to who was interred but such burial chambers “would be considered high standard monuments and so would suggest an individual or family of high status, such as the merchant class, was buried there”, Mr Noonan said. “The crypt will be unopened and remain in situ,” he added.
The archaeologists also found evidence of 18th century subterranean flues that carried heat from fires lit in the church. Further remnants depicted a later system that ferried water through earthen channels from a furnace.
The restoration work will be completed in six weeks.
Work on Youghal’s €18m main drainage system was temporarily halted after workmen, working under archaeological supervision, uncovered a full skeleton in Pax’s Lane in South Abbey.
The remains, believed to be those of a 30-year-old medieval female, were “excellently preserved by the sandy soil”, said archaeologist Daniel Noonan. “The teeth were in very good condition due to an absence of sugar additives in those times,” he noted.
Mr Noonan has complimented contractors Wills Brothers, Irish Water and Atkins engineering for fully backing the investigation. The site was once a Franciscan friary and graveyard, with links to a nuns’ community.
The skeleton “may be that of a nun but may also be from an adjoining, general burial ground”, he said. The find was logged with the National Monument database but will stay in situ.