Known locally as Portaulochain, meaning 'The Bank of the Wild Fowl', this fourteenth-century fortress was constructed by Eoghan O'Madden when the O'Maddens held considerable sway over this part of Ireland. The castle's strategic position above the river made it a valuable prize, and historical records show it changed hands multiple times; William Mac Uachtrach seized it in 1441, whilst Redmund Burke captured it during the tumultuous year of 1600.

By the time Commonwealth surveyors arrived in the 1650s, they found only 'an old stumpe of a castle close by the shannon side', suggesting the structure had already suffered significant damage, possibly during the Confederate Wars. Today, visitors will find the remnants of a rectangular tower measuring roughly 10.4 by 7.2 metres, with limestone rubble walls that once featured a slight base batter for added defensive strength. The walls now stand at about 1.7 metres high, considerably reduced from their original imposing height.

Among the rubble, one architectural detail offers a glimpse of the castle's later modifications: a portion of a doorway featuring a rebated, punch-dressed stone voussoir with reamed margins, characteristic of late sixteenth or seventeenth-century construction techniques. This suggests the castle underwent renovations during its later occupation, perhaps by the Burkes after their 1600 conquest. Though Portland Castle may lack the grandeur of better-preserved Irish fortifications, its ruins serve as a tangible link to the contested landscapes of medieval Tipperary, where Gaelic lords and Old English families vied for control of the Shannon's strategic crossing points.