Dunsink Observatory
The Dunsink Observatory is an astronomical observatory established in 1785 in the town of Dunsink near the city of Dublin, Ireland. Its most famous director was William Rowan Hamilton, who, amongst other things, discovered quaternions, the first non-commutative algebra, while strolling from the observatory into the centre of the city with some friends and his wife. He is also renowned for his Hamiltonian formulation of dynamics. In the late 20th century, the city encroached ever more on the observatory, which compromised the seeing. The telescope, then no longer state of the art, was used mainly for public 'open nights'.

History
The site was established on the south slope of a low hill in the townland of Dunsink, 84m above sea level. The South Telescope or 12 inch Grubb, is a refracting (uses lens) telescope built by Grubb Parsons, completed in 1868. The achromatic lens, actually about 11.75 or 11.8 inches, was donated by Sir James South in 1862, who had purchased the lens from Cauchoix of Paris 30 years earlier. He had intended it for a large but troubled equatorial that came to fruition in the 1830s, but was dismantled around 1838. (See also Great refractors) The entry for the observatory in Thom's Directory (1850) gives the following account of the observatory,

“ ASTRONOMICAL OBSERVATORY OF THE UNIVERSITY OF DUBLIN, DUNSINK Astronomer Royal, Sir William Rowan Hamilton, A.M., LL.D. Assistant Astronomer, Charles Thomson, esq. This Observatory, endowed by Francis Andrews, esq., LL.D., Provost of Trinity College, and erected in 1785, was placed, by statute, in 1791, under the management of the "Royal Astronomer of Ireland," an appointment first filled by Dr. Henry Ussher, and subsequently by Dr. Brinkley, Bishop of Cloyne. The Institution is amply furnished with astronomical instruments, and is open to all persons interested in astronomical science, on introduction to the resident Assistant. It is situated in Lat. 53° 23' 13" N., Long. 6° 20' 15" W. ”