While his famous name and splashy arrival generated plenty of attention, Mr. Kennedy’s job speaks to the more modest role the United States now plays in Northern Ireland. In 1998, President Bill Clinton’s envoy, George J. Mitchell, brokered the agreement that ended decades of violence known as the Troubles. Mr. Kennedy, by his own description, will function more as a cheerleader.

The United States is the largest foreign investor in Northern Ireland, with Allstate, Seagate and other companies investing 1.5 billion pounds ($1.86 billion) over the last decade. That is a fraction of the American presence in the Republic of Ireland, where low taxes and stable politics have attracted more than $350 billion.

Mr. Biden, who traveled to Belfast to mark the anniversary, left Mr. Kennedy behind in the city when he went to the south to explore his ancestral roots. Since then, Mr. Kennedy has filled his days meeting with businesspeople, entrepreneurs and the local heads of every American company with operations in Northern Ireland. He has also met with the leaders of all the major political parties.

“I am here to advocate for the people of Northern Ireland,” he said. “I am here to do that whether they have tricolor out front or a Union Jack.”

Mr. Kennedy’s ecumenical tone is no accident. His appointment last December was greeted with wariness by some unionists, who favor staying in the United Kingdom and are predominantly Protestant. They muttered that Mr. Kennedy, with his Irish Catholic roots and Irish Catholic boss, Mr. Biden, would favor the nationalists, who seek a united Ireland and are mostly Catholic. (The Democratic Unionist Party precipitated the government’s collapse by withdrawing from the Northern Ireland assembly.)

Initially, Mr. Kennedy said, he, too, worried that his name might be a hindrance. But so far, he said, he has encountered little suspicion. “People are going to project on me all sorts of things; some of them are nice, some of them are not so nice,” he said. “You have to navigate through that.”