Dunsink (Irish: Dún Sinche, meaning 'fort of Sineach') is a townland in the civil parish of Castleknock, Dublin, in Ireland. The townland has an area of approximately 1.7 square kilometres (0.66 sq mi), and had a population of 323 people as of the 2011 census. The 1880 Definition of Time Act set the official time in Ireland to be Dublin Mean Time. This was computed as the time at Dunsink, which was about GMT-25m21. In 1916, another act moved Ireland to Greenwich Mean Time. Dunsink time is mentioned five times in James Joyce's novel, Ulysses.