Born on a cattle and dairy farm in County Carlow, Ireland, he came to the U.S. as a student at Fordham Preparatory School in New York City. He became an American citizen and served in the Army Air Forces, working in intelligence in England and France during World War II. He earned a degree at the Columbia University Business School.
He became an accountant for Lybrand, Ross Brothers and Montgomery in New York and was assigned to head the Baltimore office of what had become Coopers & Lybrand in 1950. He was named a partner of the accounting firm in 1961 and a national managing partner in 1970. And while Mr. Deering maintained relationships with numerous business clients, he also gave his time to advising charitable institutions about their finances. Friends said he often used his experience in accounting to help philanthropic causes.
"He was a silent hero to many of Baltimore's nonprofits," said Alex Fisher, a friend, neighbor and business associate. "I never knew anyone so totally focused on getting it right, getting the best return for the least risk at the most effective cost. He was relentless. He had a very sharp mind."
Soon after he arrived in Baltimore, he joined the board of the Arthritis Foundation, one of the many agencies he would serve over his next five decades of public service. He was a founding member of the Greater Baltimore Committee and was an early president of the Center Club.
"He knew accounting backward and forward," said John B. Sinclair, president of the Sheridan Foundation. "He was a giant of a man who was never overbearing or arrogant. He was self-assured but modest, and an unsung hero in this community."
Mr. Deering was vice president and secretary of the Baltimore Symphony Orchestra Association. Friends said he was aware the orchestra needed its own hall in the era when it played at the Lyric Opera House. He was also a close friend and business associate of builder and developer Joseph Meyerhoff, who made a substantial donation to the construction of the symphony's present home and for whom the hall is named.
"Dad believed that civic duty was essential to a life well-lived," said his daughter, Ann Lawrence "Lawrie" Deering of Sparks. "He was always a champion of Baltimore and very involved with cultural, educational and civic organizations."
When he retired as managing partner of the Coopers accounting firm in 1982, he began a second career at Riggs, Counselman, Michaels and Downs, the Towson insurance brokers. He became its president and chief executive officer in 1984 and remained with the firm for many years.
"Pat would be aware of a need, and he had a way of seeing that something would get done, yet his name would never get near it," said Albert R. "Skip" Counselman, a friend who is chairman of the insurance firm that Mr. Deering formerly headed.
Mr. Deering was also a past president of Keswick Multi-Care and the Maryland Historical Society. He also was associated with St. Mary's Seminary & University, the Bryn Mawr School, the Kennedy Krieger Institute, Harbor Hospital, Loyola University Maryland, the Chimes and the Baltimore City Foundation.
He was also the board chairman of the Charlestown Retirement Community of Catonsville, a director of the Sellinger School of Business at Loyola University, and a past president of AAA Club of Maryland and the Green Spring Valley Hunt Club.
Friends said he had made visits to the independent schools and charities, including the Baltimore Opera Company, in the past few months.
In 2001, he established the Deering Family Foundation, which supports educational, health care and arts programs, as well as programs for the disadvantaged.
A Mass will be offered at 11 a.m. Thursday at the Roman Catholic Cathedral of Mary Our Queen, 5200 N. Charles St., where he had been an incorporator of its historic preservation trust.
His wife of 57 years, the former Mildred H.F. Foley, died in 2007. Last year, Mr. Deering married Susan Xanders.
In addition to his wife and daughter, survivors include two sons, Mark Merryman Deering of Baltimore and Charles Randall Deering of Towson; two other daughters, Kate Deering-Grieves of Baltimore and Margaret "Meg" Deering Carlson of Wellesley, Mass.; two brothers, the Rev. Mark Deering of Waco, Texas, and Michael Deering of Carlow, Ireland; two stepsons, John Xanders of Ruxton and Ted Xanders of Glendale, Calif.; 11 grandchildren; and two great-grandchildren.