Kennelly originally began his career as a pharmacist. His interest in photography led his to pursue a career in photojournalism with his wife, Joan Kennelly. Together, Pádraig and Joan Kennelly took more than 500,000 photographs of daily life in County Kerry between 1953 and 1973. The photographs have since been digitized and published as a popular book. The collection was permanently collected into the Kennelly Archive, which was launched in 2009.
In 1974, Pádraig and Joan co-founded the Kerry's Eye in the basement of their home on Ashe Street in Tralee, County Kerry. He remained the paper's editor until his retirement in 2010. Kennelly was the longest serving editor of a regional Irish newspaper at the time of his retirement. He continued to write a weekly column until his death in 2011. The newspaper has a circulation of approximately 25,000.
In early 9 May 2011, Kennelly, though in poor health, travelled to the Irish Cultural Institute in Paris for the opening of exhibition of the Kennelly's photographs documenting the 1969 visit of Charles de Gaulle to Ireland. Many of the photographs on display were taken by his late wife, Joan, including a famous photograph which was published on the front page of the Paris Match. Kennelly appeared at the opening of the exhibition with Irish Minister of State for European Affairs Lucinda Creighton and Admiral François Flohic, de Gaulle's aide-de-camp.
He died at his home in Tralee, County Kerry, on 21 May 2011, at the age of 82. He was survived by his four sons: Pádraig J. Kennelly, Jerry (founder of Stockbyte), Brendan and Kerry; sister, Eithne; two brothers, Ted and Emmet; and seven grandchildren. He was predeceased by his wife, photographer Joan Kennelly. His funeral mass was held at St John’s Catholic Church and he was buried at Rath cemetery, just outside Tralee.
- "Kerry Eye editor passes away". RTÉ News and Current Affairs. 2011-05-26. Retrieved 2011-06-03.
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- "Paris photo show recalls de Gaulle's Irish visit". Irish Times. 2011-05-09. Retrieved 2011-06-03.
- Lucey, Anne (2011-05-25). "'Kerry's Eye' founder buried in Tralee". Irish Times. Retrieved 2011-06-03.