Birr Castle (Irish: Caisleán Bhiorra) is a large castle in the town of Birr in County Offaly, Ireland. It is the home of The 7th Earl of Rosse, and as such the residential areas of the castle are not open to the public, though the grounds and gardens of the demesne are publicly accessible.
The castle grounds are also home to Ireland's Historic Science Centre, a museum of Ireland's historic scientists and their contributions to astronomy and botany.
After the death of Sir Charles O'Carroll, Sir Laurence Parsons (died 1628) was granted Birr Castle and 1,277 acres (5.2 km2) of land in 1620. Parsons engaged English masons in the construction of a new castle on the site. This construction took place, not on the site of the O'Carroll's Black Tower (since disappeared), but at its gatehouse. "Flankers" were added to the gatehouse diagonally at either side, giving the castle the plan it retains today.
After the death of Sir Laurence Parsons and of his elder son Richard, the castle passed to his younger son William. During the Irish Rebellion of 1641 William was besieged at Birr for fifteen months by Catholic forces. After the civil war, William's son Laurence (baronet from 1677) refurnished the castle.
A later descendant, The 2nd Earl of Rosse, also engaged in some re-building, and heightened and "Gothicised" the castle in the early 19th century. In turn, his son, The 3rd Earl of Rosse, was responsible for the construction of the great telescope at Birr. When completed in 1845, it was the largest telescope on earth, and capable of capturing more light and seeing further into space than any telescope had done before. Birr therefore became a focus for astronomical observations, and visitors came to visit the observatory from all over the world - including Charles Babbage and H.I.H. Napoléon Eugène, Prince Imperial.
When the 3rd Earl died, his sons carried on the scientific tradition, and The 4th Earl of Rosse is noted for measuring the heat of the moon. After his death in 1908, however, the telescope fell into disrepair; the mirror was taken to the Science Museum in London, and, circa 1914, the telescope's metal supporting structure was melted down to be used in the First World War. In 1925 the wooden structures around the walls were demolished for safety reasons. Following several intermediate restoration attempts, the telescope was restored more completely in the late 1990s.
A main feature on the grounds of the castle is the "Great Telescope" or Leviathan of Parsonstown or The Rosse Telescope) of the third Earl of Rosse, an astronomical telescope with a 183-cm (72 in) reflector. It was completed in 1845 and was used for several decades before the last observations were made in the first years of the 20th century. Its record size was not surpassed until the completion of the 100-inch (2.5-meter) Hooker Telescope at Mount Wilson Observatory in 1917. It was dismantled in 1914, but the structure was restored and the telescope reconstructed in the 1990s and is open to the public.
In 2017, LOFAR radio-telescope station IE613 was constructed in the grounds of the castle. This is the westernmost station in the LOFAR network. The I-Lofar telescope, in 2018, observed for the first time a billion-year-old red-dwarf, flare star called CN Leo, almost 75 trillion kilometres away.
Laurence Parsons, 4th Earl of Rosse and his mother, Mary Parsons, Countess of Rosse, were eminent photographers and her darkroom, which is also on show, is believed to be the oldest surviving example in the world.
The Irish entry to the 2014 European Tree of the Year contest was the Birr Castle Grey Poplar (Populus x canescens). On 12 February 2014, while voting was still open, it was blown down in a storm.