The origins of the estate lie in the sixteenth-century plantation of Munster.  William Trenchard, a Wiltshire man wishing to make his fortune, was appointed undertaker for 14,000 acres in the town lands fo Roberstown, Clonnagh, Kilmoylan, Knockpatrick and Shanagolden in 1587.  He was installed in Corgrig Castle (just east of Foynes and now incorporated into the farmyard of Corgrig House).  He retained 1,500 acres for his own use and divided the remainder among English planters.  His son, Francis, repaired the castle , increased the number of planters on the estate and obtained licenses for markets.  It is likely that the family stayed behind the thick protecting walls of Corgrig castle during the uncertain and often violent years of the seventeenth century.  Sometime during the eighteenth century, before 1767, the Trenchards decided to build a house at Cappagh, away from the nucleus of the estate at Shanagolden, overlooking the River Shannon.

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Much of the character of that house can still be seen in the three storey central five bays on the back elevation of Mount Trenchard; the rounded central bow flanked by a single bay of windows on either side.  At the base of these are two round-headed opening which suggest doors.  It is likely that the original door was placed in the centre of the bay, though possible that there were also two matching side doors in the eighteenth century house.  For initially this was the front of the house, facing the road which ran at some distance from the house from Shanagolden to Loughill, the only route to Limerick and Kerry.  The house was clearly marked on the map of the roads of Ireland which George Taylor and Andrew Skinner made in 1778.

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The hous was built from undressed blocks of roughly coursed local limestone with openings for windows and doors made from the rosy handmade bricks of the eighteenth century.  As only the cut stone string course between the first and second storey and the windowsills project from the plane of the wall it is reasonable to assume that this facade was not intended to be plastered.  There is also evidence of a sophisticated awareness of the fashionable Georgian style of the eighteenth century with its carefully proportioned windows and well formed bow, and if such a building were to be plastered the windows and corners would have been projecting and thus articulated.  The original house was almost square; another indication that an architect was involved or a builder who was aware of designs such as those published by John Payne in "Twelve Designs for Country Houses" in 1757.  There were no bows on the river front as the 1840 Ordnance Survey map shows.  This map also indicates that the house had wings which attached it to the stables and outbuildings.  They are roughly symmetrical.  This Palladian feature was popular in eighteenth century Irish houses and again points to architectural input in the design.

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The house was probably built some time between 1740 and 1760.  As Maurice Craig's survey of surviving "Middle Size" houses (into which category Mount Trenchard easily slips) shows, bows and Palladian details were an integral part of country house design vocabulary by the mid-eighteenth century.  As for an architect Francis Bindon is a possible contender.  He was born in Co Clare in c 1690 and is known to have designed houses in Co. Kilkenny, Wicklow, Clare and possibly Limerick city until 1865.  Newhall, Co. Clare, built of a mellow pink brick and incorporating a rounded bow and cut stone string course is attributed to him and is not entirely dissimilar to Mount Trenchard.  However, it is difficult to be certain about the contribution of the architect when exterior doors, the main staircase, some fireplaces and interior joinery have been replaced.  The house still has some of the fine white marble fireplaces carved with mythological figures which are similar to those that were being imported into Limerick from Italyin the late eighteenth and early nineteenth century and which can still be seen in the Chamber of Commerce on O'Connell Street.  It also has a polished grey limestone fireplace in a first floor bedroom which was made in Ireland and may have come from the Kilkenny marble works in the latter part of the eighteenth century.

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The decision to build the new house at Cappagh overlooking the Shannon rather than building up against the old castle as many families did suggests that the Trenchards were cultivated people, aware of current literary and aesthetic ideas about landscape.  The picturesque approach to the landscape design was to appreciate the inherent qualities of wild landscape, Alexander Pope's genius loci, and to endeavour to reveal and cultivate it in any design.  Ther was a bias towards the dramatic in this school of thought.  Obviously Cappagh qualified, not just because of the wide views of the river and the nearby pressence of Knockpatrick, but also for the way the river appeared to widen as you climbed the hill behind the house.  So, the house was set back from the road and instead of having terraces falling away from it in the more usual manner the full wildness and remoteness of the place was discovered as you left the house and climbed the hill behind.  It was left to descendants of the original builders to fully realise this potential with a series of terraces.  By 1852 a visitor arriving on a sunny day could appreciate the effect; " As you mount, the Shannon at every step becomes wider and grander; as you descend, its blue waters and rocky coasts become more and more beautiful"