The fortification gained official recognition in 1619 when King James I granted William Mc Brian Murtagh ownership of 'the castle and town or cartron of Donclone'. This wasn't William's only stronghold; records from 1636, following his death, reveal he also possessed another castle at Ballyknock, though that one had already fallen into ruin by the time of the inquisition.

The fate of Mosstown Castle remains something of a mystery. Local historian Bence-Jones noted in 1990 that there was a 'tower-like building' at the rear of Mosstown House, raising the intriguing possibility that the original castle had been incorporated into the later Georgian mansion. This wouldn't have been unusual for the period, as many Irish landowners chose to modernise their medieval fortifications rather than demolish them entirely.

Unfortunately, any chance of confirming this theory vanished in 1962 when Mosstown House was destroyed. Today, no visible traces remain of either the house or the castle that may have formed part of its structure. The site stands as a reminder of how quickly Ireland's built heritage can disappear, taking centuries of history with it. What was once a symbol of power in the Longford landscape has been reduced to mentions in old maps and legal documents, leaving us to imagine the tower that once watched over the lands of Dowclone.