In 1850 Ferguson was appointed clerk and secretary to a commission for arranging the records of the Irish courts, and this office he held until its abolition two years later.
On one occasion he undertook at his own expense a journey to Switzerland, in order to recover some Irish records in the collection of a Swabian baron.
He became a collaborator with William Lynch, author of Feudal Dignities in Ireland, in arranging the voluminous series of "Irish Records".
To the Topographer and Genealogist he communicated the account of Sir Toby Caulfeild relative to the Earl of Tyrone and other fugitives from Ulster in 1616; a curious series of notes on the exactions anciently incident to tenures in Ireland; a list of the castles, &c., in Ireland in 1676, with a note on hearth- money; and a singular document of 3 Edward II, relative to a contest between the king's purveyors and the secular clergy of Meath.
At his decease he left incomplete a translation of the "Norman-French Chronicle of the Conquest of Ireland", which M. Michel edited from a manuscript in Lambeth Palace Library.