The present building contains within its walls fragments of a much earlier structure, one that began as an L-shaped plantation house constructed in 1624, only a year after Arthur Forbes received his grant of 1,200 acres under the Plantation of Longford. That original building sat roughly 55 metres to the south-south-east of the Forbes castle, and it may well have been fortified, as plantation houses of the period often were, built to consolidate newly granted land in territory that remained contested. A fire in 1825 gutted much of what had survived the intervening two centuries, and the house was remodelled and rebuilt across the eighteenth and nineteenth centuries, leaving the seventeenth-century fabric embedded in, rather than visible as, the structure you see today.

Before the fire reduced it, the house had a particular character worth picturing. It rose two storeys over a basement, with hipped roofs and twin-light mullioned windows, the kind of paired stone-framed openings associated with late Tudor and early Stuart domestic architecture. A large stepped chimney on its southern side carried a series of tall diagonal chimney stacks, an arrangement that would have given the roofline a distinctive, angular silhouette. In 1682, a writer named Nicholas Dowdall described it as 'a fair and spacious house with Lovely Gardens of Pleasure', which suggests it had already acquired something of the demesne character it retains today. The first Earl of Granard, who died in 1696, had by then added an entire wing, expanding the original L-plan further. Around the house, the demesne accumulated its attendant features over time: a dovecote, a formal garden, a deer park, and an armorial plaque, a carved panel bearing a coat of arms, which survives as one of the more legible traces of the Forbes family's long occupation of this quiet corner of Longford.