After graduating at St. Patrick’s College, Maynooth, Cohalan is ordained a priest at the Cathedral of St. Mary and St. Anne, Cork on July 25, 1882. His first pastoral appointment is a curate at Kilbrittain, County Cork, from October 1883 to January 1884. He briefly resumes his post-graduate studies at St. Finbarr’s Seminary (now College), Cork, from January to November 1884. His second curacy is at Tracton, County Cork, from November 1884 to September 1896. He returns to St. Patrick’s College, Maynooth, as a professor of Theology from September 7, 1896 to June 7, 1914.
Cohalan is appointed Auxiliary Bishop of Cork and Titular Bishop of Vaga on May 25, 1914. He is consecrated bishop at the Cathedral of St. Mary and St. Anne on June 7, 1914 by John Harty, Archbishop of Cashel and Emly. Two years later, he is appointed Diocesan Bishop of Cork on August 29, 1916.
Cohalan is an outspoken critic during the Irish War of Independence, condemning acts of violence on both sides. In particular, he denounces the policy of reprisals. In July 1920, he pronounces an interdict on the killers of a Royal Irish Constabulary (RIC) sergeant, shot dead in the church porch in Bandon. He declares that anyone killing from ambush will be excommunicated. On December 12, 1920, Cohalan issues a decree saying that “anyone within the diocese of Cork who organises or takes part in ambushes or murder or attempted murder shall be excommunicated.” In turn, his life is threatened by the Irish Republican Army (IRA). In August 1928, he condemns the British government which had allowed Terence MacSwiney to die on hunger strike in 1920.
The Bessborough Home in Cork is run by the Sisters of the Sacred Hearts of Jesus and Mary and when the department “sought a change of superior in Bessborough because of the appallingly high death rate, he [Catholic Bishop of Cork Dr. Daniel Cohalan] denounced the request. The replacement of the Bessborough superior was delayed for four years after the department requested it, and many infants died during that time. It seems probable that the bishop’s intervention was elicited by the congregation.” In general the report finds that the major causes of infant mortality in the homes were respiratory infections and gastroenteritis, while “public attention has focused on marasmus [malnutrition]” suggesting “willful neglect.” However, it says that “the term marasmus is best seen as indicating that a child was failing to thrive, but medical experts suggest that this was due to an underlying, undiagnosed medical condition.”
Cohalan dies in office at the age of 94 at Bon Secours Hospital, Cork, on August 24, 1952. A story, current at the time in Cork, refers to his antipathy towards bishops of the Church of Ireland who styled themselves “Bishop of Cork.” A month before his death, and on his death-bed, word is brought to him of the death of the Church of Ireland Bishop of Cork, Cloyne and Ross, Robert Hearn. The response of Cohalan, known “affectionately” as “Danny Boy”, is reputedly, “now he knows who’s Bishop of Cork.”
Originally buried at St. Finbarr’s College, Farranferris, Cohalan is reinterred in the grounds of St. Mary and St. Anne’s Cathedral, Cork, in 1996. His nephew of the same name, Daniel Cohalan, is Bishop of Waterford and Lismore from 1943 to 1965.