The Clock Tower combined the Victorian desire for mixing the aesthetic and practical and is a well known landmark, as recorded in a local ditty:
Mister Guinness has a clock
And on its top a weathercock
To show the people Castleknock
Rising to about 37m in height, it is clearly visible above the estate's mature trees to motorists crossing the West Link toll bridge over the River Liffey or leaving the city on the road to the west. The view from its balcony is breathtaking, stretching on a clear day from Malahide to the north, Dun Loaghaire and Dublin mountains to the south, and Maynooth to the west.
It was erected in 1880 but its architect is unknown: it does not appear to have been Fuller who carried out the house renovations in the following years. It is possible that the designer was British Architect T.H. Wyatt. In his private papers Edward Cecil referred to a Mr Wyatt in connection with the bells for the Tower and to Mr Thomas Wyatt who had been recommended in connection with the house extension.
The Tower is said to have been erected by the engineering department of the Guinness brewery. The walls are 1.22m thick at ground level reducing to 0.76m at top. Limestone from a quarry across the Liffey at Palmerstown was used for the main body of the Tower and granite, possibly from the south east of the country for dressings.
The contrast between the two has been said to create a harmonious combination of both colour and texture.
The Tower contains a 8,183 litre water tank at balcony level and provided a private water supply for the estate. A weir was constructed on the Liffey in the Strawberry Beds at the location of what is now the Wren's Nest pub. A mile-long millrace brought water to turn a turbine, which in turn pumped water to the Tower and generated electricity for the house. The lines were taken across the Liffey on an iron bridge specially erected for the task. (The bridge was also used by staff at Farmleigh who lived on the south side of the river as a short cut on their way to the house.) The bridge still stands today in somewhat decrepit condition near the Angler's Rest pub.
One of two distribution pipes channelled domestic water from the tank to the house and farm. The other powered a turbine which pumped fresh water from a spring well to the house. It was a simple, yet ingenious, feat of engineering. A laundry on the south side of the Liffey may also have been connected to this system. Little evidence of this process survives apart from the Tower itself.
There are clock dials, 3.35m in diameter, on the east and west faces of the Tower below the balcony. They are cast in iron, painted and gilded, and the hands are copper. Internally, the clock mechanism is at balcony level and has three sections: the time, strike and chime sections. The time section drives a two-second pendulum, which provides the drive to the hands. The chime section controls five hammers, which strike four bells on the quarter hour to play the Westminster chime. The strike section controls one hammer and strikes the largest of the five bells on the hour only.
The bells were cast in 1879-80 and each one bears a Latin inscription. The weights for the different trains along with pendulum are suspended in the stairwell. At the bottom of the stairwell there is a large timber container of sand to protect the weights in the case of a fall.
The elaborate clock mechanism was manufactured by Sir Howard Grubb, the famed instrument maker, who had premises at 51 Rathmines Road in Dublin. Grubb more usually made clockwork drives for his equatorial telescopes and this is the only known tower clock by him. He supplied all of the instrumental equipment for the observatory in University College Cork, the great roof at the Dunsink observatory, as well as domes of the Imperial and Royal Observatory in Vienna. He also supplied a telescope for the Clock Tower in 1882 at a cost of £90. Grubb installed the clock mechanism in the tower about March 1885. Its restorer believes that it is a unique design.
The clock remains in perfect working order after more than a century and, until recently, it was wound every day by hand. The weights are now raised electrically but there has been no alteration to the clock itself.