What most visitors do not realise is that the fort replaced something far older and considerably grander: a substantial country house that had stood on the same commanding rise for well over a century before the military engineers arrived.

The house was erected in 1611 by Sir Edward Fisher, who chose the hill for the kind of reason that always appealed to men of his era, namely visibility and prestige. By 1618 Fisher had surrendered his position to the Crown, and the property passed into royal hands as a residence for the king's representative in Ireland. From 1617 onwards it was recorded as His Majesty's House at Kilmainham, and referred to simply as the Phoenix, a name that would eventually attach itself to the entire park. The house had a working life that stretched across some turbulent decades. In the 1650s Henry Cromwell took up residence there and added a large wing, several storeys high. A chapel and hall followed in 1661. The Down Survey of 1655 to 1656, a remarkable mapping project carried out under Cromwell's administration, recorded the parish of Kilmainham and shows a large gabled building with three chimneys occupying the hill. That image is now among the few traces of the place. By 1734 the house was gone, demolished to make way for the Magazine Fort, a powder and arms store whose construction drew immediate satirical fire from Jonathan Swift. His verse on the subject has survived rather better than the building it mocked: 'Behold a proof of Irish Sense, here Irish wit is seen, when nothing's left that's worth defence, they build a magazine.'

Thomas's Hill is accessible within the Phoenix Park, and the Magazine Fort, though no longer in military use, is a visible landmark on the rise above the Liffey. There is nothing above ground to indicate the earlier house, but the earthwork outline of the fort does follow the general footprint of the hilltop that Fisher first built upon. The Down Survey parish maps are available to view online through the relevant Irish heritage databases, and the Kilmainham sheet gives the clearest contemporary impression of what the vanished building actually looked like. Standing on the hill and looking back towards the city, it is not difficult to understand why the site appealed to a seventeenth-century gentleman, or indeed to a Cromwellian administrator, or eventually to an army ordnance department.