Early 17th-century maps of Ardagh barony mark this spot with a castle-type building, labelled as the 'Castle of Keillen' on the Ordnance Survey Fair Plan. Today, nothing remains visible above ground; the site has been transformed into a working farmyard with a roadway cutting through what was once the castle's footprint.

The castle's history becomes clearer in 1682, when Nicholas Dowdall recorded that 'George Cunningham Esqr. Hath built a fair house at Killinlosorogh which signifies a flaming wood'. This intriguing place name, meaning 'flaming wood', hints at some forgotten local legend or perhaps a historical event that gave the area its distinctive identity. Cunningham's 'fair house' likely replaced or incorporated the earlier castle structure shown on those 17th-century maps.

Like many of Ireland's lesser-known castle sites, Killinlastra has vanished completely from the landscape, levelled at some unknown date and absorbed back into agricultural use. What was once a symbol of power and prestige in rural Longford has become another ghost site, its existence preserved only in old maps, historical documents, and the careful cataloguing work of archaeologists like Caimin O'Brien, who compiled this site's record in November 2012.