I have located Ballacostain on my reproduction first ed. ordinance survey map (circa 1874). It is just down the hill from Balnahow, in the little valley where the railway line now runs. I lived in this area in my formative years in the 1950s and 60s and there was no house or farm there then. Maybe it was obliterated when they built the railway. Perhaps an IOM Railway enthusiast can throw some light on it.