More than 350 landmark buildings across the world took part in the celebration.
These included the Rockefeller Centre, the top of the Rock Observation Deck and Madison Square Garden in New York City, the Hotel de Ville in Paris, the famous Tokyo Tower in Japan, Canada’s CN Tower and the Sydney Opera house.
At home there were events all over the country to mark the day and Minister for Health Dr James Reilly attended the formal blue-lighting ceremony at Hotel Meyrick in Eyre Square.
Autism support groups called on people to wear blue, paint their nails or dye their hair blue and light blue lights in their homes.
It was the fifth annual World Autism Awareness Day.
The United Nations General Assembly unanimously declared 2 April as World Autism Awareness Day to highlight the need to help improve the lives of children and adults who suffer from the disorder so they can lead full and meaningful lives.