Dennis O'Neill was born in Chicago, Illinois, in 1947. He received his education in the seminary system of the Catholic Archdiocese of Chicago and was ordained a priest in 1973. He graduated with a Bachelors Degree in English Literature from Loyola Universtiy in 1969 and a Masters of Divinity from St. Mary of the Lake Seminary, Mundelein, Illinois, in 1973. In 1974, he received an S.T.B and an S.T.L. from St. Mary of the Lake Seminary. Since ordination, he has served in four parishes in Chicago and is currently pastor of St. Martha Parish, in Morton Grove, Illinois.
He is author of Lazarus Interlude: A Story of God's Love in a Moment of Ministry (Notre Dame, IN: Ave Maria Press, 1982). And he has written either the introductions of the texts for several books published by British Celtic Artist, Courtney Davis: the Introduction to Celtic Illumination: the Irish School New York: Thames & Hudson, 1998); co-authored St. Patrick: A Visual Celebration (London: Blandford, 1999); and Celtic Beasts (London: Blandford, 1999), and authored 101 Celtic Crosses (Newton Abbot, Devon: David & Charles, 2004).