Original sentence: It is chiefly known as the location of the headquarters of the Irish Air Corps (Irish Air Defence Forces) at Casement Aerodrome, Baldonnel, which has been in operation for over 90 years in the locality, first as an RAF base until the creation of the Irish Free State in 1922, when the Irish Air Corps took command of the field
Edited sentence: It is chiefly known as the location of the headquarters of the Irish Air Corps (Irish Air Defence Forces) at Casement Aerodrome, Baldonnel, which has been in operation for over 90 years in the locality, first as an RAF base during the occupation, until 1921/2, when the Irish Air Corps took command of the field