When the Sisters Of Mercy came to Castlebar in 1845, they bought a house known as The Grove on Lower Charles Street. They then acquired Rock House, just off The Mall in 1853, which had been Lucan's Bank, and set up a secondary school there and called it St Joseph’s Secondary School. Because of the big demand for secondary education, the nuns bought the residence of Lord Lucan, which was behind their school at Rock House, and the 92-acre demesne for the sum of £2900. The year was 1924. The land was referred to as The Lawn and the house was Castlebar House but locals when talking about the house now refer to it as The Lawn which encompasses the complete estate.
At the time, the nuns converted the house into boarding accommodation for students attending their nearby convent. The Lawn House was gutted by fire in 1935 but was rebuilt after retaining its original architectural beauty at a cost of £10,000. The boarding school was closed in 1974 and the building is now the Secondary School. Then in 1982, a large extension was built on to the school.
On display inside the school is a most interesting decorated scroll. This is known as the 'Lucan Scroll'. It is the welcoming address given by the Castlebar people to Earl and Countess of Lucan when she visited her ancestral home in 1889. This 'Address of Welcome' was signed by 45 dignitaries of the town at the time.
In 1918, the Board of Management (which was introduced in 1985) organised a hugely successful Centenary Celebration weekend.