There has been a castle on the site since 1170, and from the 14th to the 17th century the O'Carroll family ruled from here over an area known as "Ely O'Carroll".
After the death of Sir Charles O'Carroll, Sir Laurence Parsons 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, Laurence Parsons, 2nd Earl of Rosse, also engaged in some re-building, and heightened and "Gothicised" the castle in the early 1800s. In turn, his son, William Parsons, 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 the observatory from all over the world - including Charles Babbage and Napoléon Eugène, Prince Imperial.
When the 3rd Earl died, his sons carried on the scientific tradition, and the 4th Earl (Lawrence Parsons, 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.
The "Great Telescope" and other features
A main feature on the grounds of the castle is the "Great Telescope" or Leviathan (aka 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 254-cm (100-in) Hooker Telescope in 1917. It was dismantled in 1914, but was restored in the 1990s and is open to the public.
Laurence Parsons, 4th Earl of Rosse and his mother were eminent photographers and her darkroom, which is also on show, is believed to be the oldest surviving example in the world.
The grounds of the castle contain the oldest wrought-iron bridge in Ireland, dating from 1820.
The walled gardens in the grounds feature Box Hedges that are over 300 years old. They are also , according to The Guinness Book of Records, the tallest hedge in the world.