The fortress consists of two quadrangular towers connected by a central court, with the entire complex reaching heights of around 45 feet. The southern and northern towers each measure roughly 20 feet by 8½ feet internally, containing four storeys in total; two vaulted floors topped by two additional levels. Between them, the court spans 32 feet by 20 feet and rises through three storeys, its walls a sturdy 3½ feet thick. Archaeological evidence suggests the western side incorporates elements of a 13th century hall house, complete with a battered base and later modifications including inserted crosswalls and vaulting.

The castle's history reads like a chronicle of Irish power struggles. Originally attributed to Cormac Mac Eniry, chief of the ancient Corcomohid clan who supposedly built it during King John's reign, the fortress served as one of their strongholds alongside their principal seat at Castletown Mac Eniry. The Mac Eniry ownership ended during Elizabeth I's reign when the castle was forfeited, eventually passing to the Billingsley family in 1588. By 1604, Sir John Dowdall had secured it as part of his wife Elizabeth's jointure, setting the stage for one of the castle's most dramatic episodes.

The Confederate Wars brought Kilfinny's finest hour of defiance when Lady Dowdall found herself besieged within its walls for forty weeks in 1641-42. The Confederate forces under Eady Lacy of Bruree initially attempted ambush tactics, only to be repelled by ten musketeers firing from a windmill. General Purcell's subsequent assault saw the mill and barns taken and then lost again when defenders sallied forth and burned the buildings. Even after the sheriff Richard Stephenson was shot during another attempt, and protective "sows" (mobile siege shelters) were pierced through, Lady Dowdall managed to relieve nearby Croom Castle five times despite the blockade. Only when Limerick Castle's surrender provided the Confederates with heavy artillery did Purcell return with overwhelming force; after just three cannon shots, Lady Dowdall finally surrendered, though Lord Inchiquin ensured her safe passage. The castle later passed to the Piggott family, with Lieutenant Colonel William Piggott recorded as owner in the 1650s Civil Survey, which noted the property included not just the castle but also an orchard and mill seat.