For probably the first time in history, there were two US presidents on Irish soil at the same time on the same day.
Mr Obama arrived in Belfast where he addressed 2,000 students before heading to the Lough Erne Resort near Enniskillen where the summit is taking place.
Meanwhile, almost 300km away in Co Clare, Mr Clinton, who is no stranger to Shannon, made a brief stopover at the airport on his way to the Middle East.
Mr Clinton went for a "good walk-around" the terminal building while his aircraft was being refuelled. He also enjoyed refreshments in the airport's VIP suite and strolled around the duty free shops.
In 1998, during his second term as president, Mr Clinton arrived in Shannon on board Air Force One and addressed huge crowds in Limerick city centre.
Later that year, while travelling back to the US from Bulgaria, Mrs Clinton spent several hundred pounds in the duty free stores at Shannon.
Mrs Clinton, who is considered to be a potential 2016 White House contender, wrote in her book 'Living History': "Of all the places we visited during the eight years of Bill's presidency, none was more invigorating and inspiring than Ireland."