Mongavlin Castle, Medieval fortress on River Foyle in County Donegal, Ireland

Mongavlin Castle is a ruined lowland castle on the west bank of the River Foyle, about 3 km south of St Johnston in County Donegal. Built of lime and stone, it measured 50 by 25 feet and stood three and a half storeys high, with four flankers at the top and an iron portcullis. Only a small portion remains standing.

Mongavlin was a stronghold of the O'Donnells and the chief residence of Iníon Dubh, mother of Red Hugh O'Donnell, in the 16th century. The present ruin was built for Sir John Stewart of Methven; a 1622 survey records a lime-and-stone castle on the Foyle, 50 by 25 feet and 3½ storeys, with four flankers and an iron portcullis. The O'Donnells abandoned it after their infighting around 1608.

Mongavlin takes its name from the Irish Maghgaibhlin, and the castle is also recorded as Mongevlin in state papers and surveys. Its sixteenth-century role as Iníon Dubh's chief residence left a mark on the district: the Crawford bodyguards she brought from Scotland settled here after the O'Donnells abandoned the site, and many of their descendants still live in the area.

The site lies in a rural area south of Saint Johnston village and is reached by country roads. There is little infrastructure nearby, so it is worth coming prepared with suitable clothing for the weather.

The flagstone over the archway bore the inscription 'J.S.-E.S.T.-1619', linking the building to Sir John Stewart and his wife Elizabeth. It disappeared in the early eighteenth century, so visitors today see the archway without its original carved date stone.