Dunsink Observatory, located near Castleknock on the outskirts of Dublin, opened its doors in 1785 and is Ireland’s oldest observatory. Originally built for Trinity College Dublin, it is now part of the Dublin Institute for Advanced Studies (DIAS). From the beginning Dunsink Observatory has been used to look outwards into space, and it hosts the 150-year-old South Telescope that can still be used on clear nights to view stars and planets. The Observatory has a long history of research in astronomy and astrophysics, both earth-based and space-based. Dunsink’s most famous resident, William Rowan Hamilton, was a brilliant mathematician and his discovery of Quaternions has found many modern uses including spacecraft control and computer graphics. Nowadays, public open nights happen regularly during the winter months, as well as educational visits for schools and other groups.