The water from the roof drains into an underground tunnel. In order to prevent flooding after very heavy rain they installed a French drain. This meant that when the water reached a certain level it drained off through a channel. The staff used the rainwater for the kitchen.
There a number of tunnels still in existence. There is one which leads to cold storage rooms. In the days before refrigerators all big houses had underground rooms to keep the food cool particularly in the summer. There is also a well which supplied fresh water to the Casino. One of the tunnels no longer in existence is the one which joined the main house in Marino to the Casino. This tunnel went from what is now the gents toilet and was about half a mile long. At intervals along the tunnels there were grills on the roof which allowed the light to penetrate.
It is of three stories although externally it appears to be only one. There is also access to the flat roof from the upstairs. Charlemont used to bring his guests up on to the roof in the warm weather where he used to have a type of marquee erected. You can still see the rings on the walls where he used to have the poles for the canvas covering. They used to have picnics there and enjoy the magnificent views of Dublin Bay, the city and the mountains.
The expensive tastes of The Earl of Charlemont put a huge strain on his finances. He had to sell off much of his land in the north to pay his bills. In 1775 he had a financial crises brought on by the expense of building the Casino. He was forced to adopt strict control of his spending. He even kept sheep at Marino which he sold as mutton.
On January 1777 when all the building at Marino had ceased, Sir William Chambers presented his final bill for thirty seven pounds. The bill had not been paid by May 1779 when Chambers wrote again and added "Some time ago I sent over a bill for some designs done for your Lordship from Jan. 1773 to Aug. 1775 amounting to £37: 5 :6 which I presume has been forgot. If your Lordship desires a fresh copy I will send it over or may I take the liberty of drawing upon you for the Amount" Charlemont had not forgotten the bill. In fact he queried it, in two items. On the reverse of Chambers bill, is Charlemont’s comment. "There is, I believe a mistake in the charge of £8: 8: - for various designs for flower stands. There are but two such designs exclusive of that part which is afterwards charged £3: 3: - and which alone answered my purpose, the others being only unsuccessful attempts, which if it were intended that they should be charged, as I rather suppose it was not, ought not certainly to be charged at a higher price than that design alone which was of any sort of use to me"..
This sort of examining bills gives us some idea as to how hard up Charlemont had become from the days of sending ships full of art and books to Ireland from the continent with no thought of cost and then paying large sums to have them stored. It is of three stories although externally it appears to be only one.