It's easy to spend half a day exploring the attractions and gardens of Birr Castle demesne. The castle dates from 1620 and is a private home, but during May to August you can visit the main living quarters on tours (which must be booked in advance). Most of the present building dates from around 1620, with alterations made in the early 19th century.
The 50-hectare castle grounds are famous for their magnificent gardens set around a large artificial lake.
The gardens are home to more than 1000 species of plants from all over the world, so there's invariably something in bloom. Look for one of the world's tallest box hedges (which has made the Guinness Book of Records), planted in the 1780s and now standing 12m high, and the romantic Hornbeam cloister. There are waterfalls, wildflower meadows and a pergola festooned with a wisteria planted in 1936.
The Parsons clan, who have owned the castle since 1620, are a remarkable family of pioneering Irish scientists, and their work is documented in the historic science centre. Exhibits include the massive telescope built by William Parsons in 1845. The 'leviathan of Parsonstown', as it was known, was the largest telescope in the world for 75 years and attracted a wide variety of scientists and astronomers. It was used to make innumerable discoveries, including the spiral galaxies. After the death of William's son, the telescope slowly fell to bits. It has been completely restored, however, and may be viewed in all its glory in the gardens.
William Parsons' wife, Mary Ross, was a keen photographer and her dark room was reputed to be one of the first of its kind in the world. You can now view a replica. Other highlights are a children's adventure playground, complete with playhouse, hobbit huts and trampolines, and the excellent Courtyard Cafe (open daily March to October; weekends only November and December), which showcases local produce. Otherwise you can wander the garden to your heart's content, with over 8km of trails including short walks.