Taoiseach Enda Kenny has delivered the commencement address to more than 20,000 graduates of Boston College.
About 50 people protesting against abortion greeted Mr Kenny as he arrived.
One group, Students for Life, accused Mr Kenny of being the biggest supporter of abortion in Ireland.
Their cause was given a boost by the decision of Cardinal Seán O'Malley to boycott proceedings. He was due to attend.
However, Mr Kenny was given a warm welcome by the vast majority of people at Boston College.
They included the Jesuit priest who is the president of the college, Fr William Leahy.
Another Jesuit priest led the prayer in advance of the Taoiseach's speech.
During his address, Mr Kenny referred to the Boston Marathon bombing, in which three people were killed.
He said the hurt of the attack was still palpable, but the people of the city had responded with what he said was their usual courage, dignity and compassion.
Cardinal O'Malley of Boston earlier said he would not attend the event because of, what he said was, the Taoiseach's support for abortion legislation in Ireland.
The Taoiseach also received an honorary doctorate.