He was born in Tubrid, Ardfert, County Kerry, to Patrick O'Donnell of Tubridmore, and Bridget (née Griffin) of Lerrig. He was a grandson of John O'Donnell of Ardfert, who lived at Tubridmore, and descended from O'Donnells of Tyrconnell, following on the implantation of O'Donnells in Ardfert by Prince Hugh Roe O'Donnell en route to the Battle of Kinsale in 1601, as recorded in the Annals of the Kingdom of Ireland. He wrote poetry as a hobby. He initially farmed a family farm at Tubrid until he left to Ballyhaise, an agricultural college.
He married Hannah Leane, from Ballintobeenig, and they had one child, Patrick Denis O'Donnell. He died of an accident when his son Patrick was only 11 years old, and his widow, Hannah, subsequently emigrated to the United States of America at the outbreak of World War II, when their son, Patrick, joined the Irish Defence Forces.
Denis O'Donnell was schooled in Tubrid, in Spa, and later in Chapeltown national schools, and then farmed some years before proceeding to study and become one of the first graduates of Ballyhaise College, County Cavan, Ireland, with a Diploma in Dairy Science, having studied there from 1908 to 1910. This was the only such qualification available in the country at the time.
In his early career he was manager in some small creameries, one in County Tipperary, and others in Black Abbey, Adare, County Limerick; in Dicksgrove, Farranfore, and BallymcElligott, and Ballydwyer in County Kerry. He was influenced by the cooperative movement being driven by Sir Horace Plunkett, well described in the account of country life of the times by Elizabeth, Countess of Fingall, Seventy Years Young. Sir Horace Plunkett, MP, and later Senator of the Irish Free State, third son of Lord Dunsany, was father of the Irish agricultural co-operative movement, and founded the Irish Agricultural Organisation Society (now the Irish Co-operative Organisation Society), which included several hundred creameries. Accordingly, Denis O'Donnell actively organised farmers in the early cooperative movement in County Kerry, and founded the Lee Strand Co-operative Creamery on April 20, 1920 in Church Street, Tralee, still a going concern in 2009 although it relocated to Ballymullen in 1992. He owned a pub at #2 Edward Street, Tralee, run by his sister Bride, until her emigration to Kimberly, South Africa. The pub was sold in 1938. He also owned shops in Rock Street and Castle Street in Tralee. He was the first to introduce the pasteurisation of milk, much against the prejudice of dairy farmers at the time, and also was the first to market ice-cream in Tralee.
His entrepreneurial streak and outreach to farming communities provided a socio-economic complement to the political mobilisation undertaken by his kinsman, Thomas O'Donnell, M.P. for West Kerry, who campaigned for agrarian reforms. Maurice Moynihan, father of Maurice Gerard Moynihan, was a member of the Irish Republican Brotherhood and the Gaelic Athletic Association, was Thomas O'Donnell's campaign manager and was a cousin of Denis O'Donnell.
- Writer recalls Tralee youth, interview with Patrick Denis O'Donnell (Denis's son), published in The Kerryman, August 15, 1975
- Early records of the Lee Strand Co-operative Creamery including documents signed by Denis O'Donnell, e.g. such as relating to a special general meeting on 5 July 1926 (Family Archive of the O'Donnells of Ardfert).
- Annals of the Kingdom of Ireland (Annála Ríoghachta Éireann) by the Four Masters, from the earliest period to the year 1616, compiled during the period 1632-1636 by Brother Michael O’Clery, translated and edited by John O'Donovan in 1856, and re-published in 1998 by De Burca, Dublin.
- The Life of Hugh Roe O'Donnell, Prince of Tyrconnell (Beatha Aodh Ruadh O Domhnaill) by Lughaidh O'Cleirigh. Edited by Paul Walsh and Colm Ó Lochlainn. Irish Texts Society, vol. 42. Dublin: Educational Company of Ireland, 1948 (original Gaelic manuscript in the Royal Irish Academy in Dublin).
- A Political Odyssey - Thomas O'Donnell, M.P. for West Kerry, by J. Anthony Gaughan, Kingdom Books, Dublin, 1983 [ISBN 0-9506015-4-3]. Gaughan is Chairman of the National Library Society of Ireland.
- Seventy Years Young, Memoires of Elizabeth, Countess of Fingall, by Elizabeth Burke Plunkett, Lady Fingall. First published by Collins of London in 1937; 1991 edition published by The Lilliput Press, Dublin 7, Ireland [ISBN 0-946640-74-2]. This Elizabeth, was a Burke from Moycullen in County Galway, who married the 11th Earl of Fingall, and should not be confused with Elizabeth Plunkett (née FitzGerald), Baroness Killeen, first wife of Luke Plunkett, the later 1st Earl of Fingall (1604-1611).