The Industrial and Commercial Property (Protection) Act, 1927 gave the NLI legal deposit status for the first time. Richard J. Hayes became Director in 1940, the title having changed from Librarian to Director in 1934/35. His contribution to Irish bibliography is unequalled; it includes Sources for the history of Irish civilisation: Articles in Irish periodicals; Manuscript sources for the history of Irish civilization, and (compiled along with his colleague Brighid Ní Dhonnchadha) Clár Litridheacht na Nua-Ghaedhilge 1850-1936.
The severe shortage of space was the main problem which faced Patrick Henchy, who succeeded Hayes as Director in 1967. In 1970/71, a report prepared by Dr Kenneth Humphreys, Librarian of the University of Birmingham, dealing with accomodation, staffing and other issues, and never published, was submitted to the Minister for Education. As a result, further premises in the vicinity - notably nos. 2 and 3 Kildare Street (previously part of the premises of the Kildare Street Club), which now house the Manuscripts Department, the Office of the Chief Herald and an exhibition area, and nos. 4 and 5 Kildare Street, were acquired and a commitment made to develop the Library on the Kildare Street site. This did not happen immediately. The accommodation problem and the inadequate purchase grant were the recurrent concern of Alf MacLochlainn, who served as Director from 1976 to 1982, and his successor Michael Hewson. Refurbishment of the Kildare Street premises began in 1982, and the Manuscripts Reading Room was opened in 1990.
In July 1986, the Library was transferred to the Department of An Taoiseach. The Department of Arts, Culture and the Gaeltacht (now Arts, Sport and Tourism) was established in 1992, and assumed responsibility for the Library. Patricia Donlon became the first female Director in 1989. Dr Donlon retired in February 1997. Seán Cromien then held the post of Director in a part-time acting capacity until September 1997, when Brendan O Donoghue was appointed to the post. During his six years as Director, staff numbers increased significantly, as did funding for acquisitions. The National Photographic Archive, housing the Library's photographic collections opened in Meeting House Square, Temple Bar, in October 1998. The Leinster Lane premises, formerly part of the National College of Art and Design, have been converted and now house the exhibition area, a lecture theatre and the Prints and Drawings Department.
Brendan O Donoghue retired in September 2003, and was succeeded by Aongus Ó hAonghusa (to 2010) and Fiona Ross (to 2014). The National Cultural Institutions Act, 1997, altered the statutory and administrative framework of the Library, giving it greater autonomy, with a Library Board replacing the Council of Trustees.