- There are 80 letters either to or from John O’Donovan O’Donovan’s principal correspondent is William Reeves.
- One letter is to George Petrie [1789-1866).
- Three letters are to James Henthorn Todd (1805-1869).
- One letter is to Edmund Getty One letter is to O’Donovan from Arthur Blennerhassett Rowan (1800-1861).
- One letter is to O’Donovan from W. Oldham.
The correspondence with William Reeves often takes an interesting form. Reeves writes to O’Donovan with enquiries about Irish words and place-names and O’Donovan annotates the letter with his answers and sends it back.
In addition to these letters, there are 26 fragments, mostly of questions and answers; some of these may belong with letters, as there is sometimes reference in a letter to queries enclosed.