(UNDATED) The patron saint of Ireland graces America’s most celebrated cathedral, and the country’s flagship Catholic university calls its student athletes the “Fighting Irish.”

Somewhere between these poles of piety and pugnacity, between St. Patrick and the University of Notre Dame, rests the soul of Irish Catholicism — and, by extension, the soul of the American Catholic Church.

For more than a century, being Catholic meant you were Irish almost by default. The Irish manned the hierarchy, built the parishes, and kept the schools and hospitals running.

“The Irish dominated the church,” said Jay P. Dolan, a former history professor at Notre Dame and expert on the Catholic Church in America. “They set the mold for how Catholicism would play out.”

By most accounts, the outsized life of the late Sen. Edward M. (Ted) Kennedy fit comfortably in that mold. He seldom backed down from a scrap — even with his own church — and battled ceaselessly for the underdog. But he could also be a man of quiet prayer who looked to Catholic priests for guidance, particularly when tragedy made one of its frequent intrusions into his life.

“He continued to have a relationship with the church,” said veteran Catholic journalist David Gibson. “It was sometimes combative and sometimes tenuous, but always very deep. The culture and the rituals were instilled in him. … They carried him through.”

In some ways, the Kennedys embodied the Irish Catholic experience in America. They could be boisterous in public, but often worshipped quietly. They were fiercely loyal to their clan and to the church — if not always to its teachings. They revered priests, identified faith in God with good works on behalf of others, and some, like the senator, were said to attend Mass regularly.

In a book published in 2008 called “Being Catholic Now,” the late Sen. Robert Kennedy’s daughter, Kerry, wrote: “Catholicism was integrated into every aspect of my family’s intellectual, moral, social, cultural, political and spiritual life. It was impossible to separate the influence of the church from that of our Irish heritage or Democratic politics.”

Countless other Irish Catholics have embodied that same trinity, though in recent years some have defected to the Republican Party over abortion and other issues. Still, the link between religion and ancestry remains so strong that the phrase “Irish Catholic” has begun to seem almost redundant.

“Irish Catholicism is so engrained in people’s minds that the two simply go together,” said Terry Golway, who teaches history at Kean University in Union, N.J. Growing up, some Irish Catholics were surprised to hear the rubric “Roman Catholic” applied to them; they thought it referred to Italians.

Persecuted by the English and starved by the potato famine, Irish Catholics were one of the few waves of immigrants to bring their own priests when they landed on U.S. shores. Unlike the clergy in other countries, many of whom sided with the ruling powers, Irish priests were considered heroes for battling fiercely against the colonizing British Protestants.

Once in the U.S., Irish Catholics established separate schools, orphanages, hospitals and political machines, protecting the flock from Protestant nativists. “Irish Catholics had to fight for a place in this country,” said Sister Mary Ann Walsh, a spokeswoman for the U.S. Conference of Catholic Bishops, “but they rose very quickly with the church’s support.”

In return, priests, and the church in general, were revered.

“Irish Catholics were intensely loyal to church,” said James O’Toole, a history professor at Boston College. That fealty meant you were in the pews for Mass every Sunday, you celebrated each sacrament, and you obeyed your priest, he said.

“Irish Catholics became more Catholic than the pope,” said Gibson. “They were so clerical and observant of church rituals.”

Despite the well-known Irish predilection for carousal, a certain gravity pervades their worship, said Bill Donohue, president of the New York-based Catholic League, which keeps an eye out for anti-Catholicism.

“When it comes to religion, the Irish are much more somber, more solemn,” he said. “The animation you would find in a pub, for example, does not carry over into church.”

Some cultural historians say the Irish are responsible for the stereotype of the guilt-ridden Catholic, who can’t even enjoy his favorite sins.

In any case, many Irish Catholics have, like the Kennedys, assimilated into American culture, and that, along with changing attitudes towards church authority, has nearly obscured the markers that define Irish Catholicism. Increasing waves of Hispanics, in turn, have obscured the markers that define American Catholics.

Author Alice McDermott, whose novels delicately trace the lives of Irish Catholics from weddings to wakes, has said: “I think there’s more and more a sense of a culture the like of which we will not see again.”

In some ways, though, Irish Catholics continue to hold court. The surnames of bishops in the U.S. still read like the roster of a Gaelic football team: O’Malley, Mahony, Dolan, Farrell, O’Brien.

“You look out at the bishops’ conference,” said Gibson, “and half the bishops look like Ted Kennedy.”