Twenty years of peace-building have come to fruition with the Queen's visit, a former Irish president said.
Dr Mary Robinson lauded the contribution of the Northern Ireland political process which ended violence in 1994 and said without it today would not have happened.
She was invited to Cambridge to accept an honorary degree in 1991 and two years later had tea with the Queen in an unprecedented diplomatic breakthrough.
The former UN human rights high commissioner said today was a good day for relations between Britain and Ireland and praised the work of her successor President Mary McAleese.
"It is wonderful that now it is coming to fruition for both islands," she said.
Dr Robinson acknowledged security concerns and said it would have been good if the Queen could have met more members of the public. She guided the Queen around Trinity College as chancellor of the historic institution.
"Security has to be taken seriously and there were sufficient indications that there might be a problem," she said.
"For the relations between our two countries this is a very good visit."
Dr Robinson said the royal event needed the peace process.
"We could not have had a visit like this without the peace process," she added.