From the Stowe 1848 Sale Catalogue:

The Manuscript Library is fitted up entirely in the gothic style, by the late Sir John Soane, who was the architect employed, and whose designs for the decoration of this room were correctly modelled from the ornaments of Henry VII’s chapel in Westminster Abbey. In the centre of the vaulted ceiling is a circular shield, filled with armorial bearings, seven hundred and nineteen in number, of the Grenville, Temple, Nugent, and Chandos families. The window frames are carved in oak, and glazed with stained glass borders; the book-cases are in bronze frames; and the chimney piece is of black marble, with appropriate or-molu ornaments. This room formerly contained a most valuable collection of Manuscripts, consisting of about two thousand volumes, classed according to their subjects, viz.: Ancient Irish Manuscripts; and others relating to the History of Ireland; Saxon and Norman Charters, from the year 697; Ecclesiastical and Topographical, Parliamentary and Political; including a large number of original Letters and State Papers, from the reign of Henry VII to that of George III; Heraldic and Genealogical, comprising a most excellent series of Grenville and Temple Family Evidences, from the reign of Henry III, and altogether forming a most important treasure of English and Irish historical and political learning, equally interesting and valuable to the Historian and Antiquary. The collection includes that of the late Thomas Astle, Esq., [1705–1803] Keeper of the Records in the Tower of London; and the valuable Manuscripts formerly belonged to Charles O’Conor, of Belanagare, the Historian of Ireland.

His grandson, Dr O’Conor, the venerable, amiable, and excellent librarian of Stowe, here passed the days of his learned age, surrounded by the inestimable literary riches of his native land, the earliest western station of learning and revealed religion. This elaborate work, in four volumes, quarto, composed by this revered and learned man, in this library, and privately printed at Buckingham, entitled “Rerum Hibernicarum Scriptores Veteres,” will hand his name down to posterity, whilst it preserves from oblivion, and the hand of Time, the chronicles and records deposited in this room, the translation of which, contributed to compose that great national undertaking. The Catalogue Raisonné of the Manuscripts in this library, also written by Dr O’Conor, is a splendid monument of education and research. It is in two volumes, quarto, and was privately printed at Buckingham. Both these invaluable works were compiled and printed at the sole expense of the late Duke of Buckingham and Chandos, at a cost of many thousand pounds. They have been entirely confined to gratuitous distribution among his Grace’s friends, and to all the principal libraries of in Great Britain and Ireland, and in all the cities of Europe. This noble munificence on the part of his Grace has been scarcely yet known, or duly appreciated, by his contemporaries; but posterity will, no doubt, do ample justice to his memory. Dr O’Conor retired to Belanagare, his native place, in Ireland, in 1826, and was succeeded in his office of Librarian, by Mr William James Smith, who from that period, until the recent dispersal of the Stowe property, has had the care of the literary treasures contained in this Library. The whole of these Manuscripts are about to sold by Messrs. Sotheby and Co.