Castlemaine, or Caisleán na Mainge as it's known in Irish, meaning 'the Castle of the River Maine', stood in the parishes of Kilcolman and Kiltallagh within the barony of Trughanacmy. Built in 1215 by the sons of Maurice Fitzgerald, this fortress represented the ambitious reach of one of medieval Ireland's most powerful families. The Annals of Inishfallen, one of Ireland's most important historical chronicles, specifically records that year: 'The son of Maurice Fitzgerald built a castle at Mogh Laithibh and Caluith na Feirrse.'

The castle's strategic position was remarkable; according to historical accounts, it actually spanned the River Maine itself, though historians have debated the exact nature of its construction. The 18th-century historian Dr Charles Smith claimed the castle was built directly on the bridge, though this assertion was met with scepticism even by Victorian-era scholars. By 1841, when John O'Donovan conducted his work for the Ordnance Survey, the castle had completely vanished from the landscape. O'Donovan noted that whilst no physical traces remained, local memory preserved its location at what was then called the Bridge of Castlemaine, and he recommended marking the site on official maps to preserve its historical significance.

Today, visitors to Castlemaine won't find towering walls or crumbling battlements, but the site remains an evocative reminder of how the Anglo-Norman conquest reshaped Ireland's physical and political landscape. The area also contains another lost fortress site at Caladh na Feirse (Callownafersy), built by John Fitzgerald in the same year as Castlemaine. These phantom castles, existing now only in place names and historical records, tell the story of how the Fitzgeralds, later Earls of Desmond, first established their power base in Kerry; a dominance that would shape the region's history for centuries to come.