Air India's Boeing 747 Kanishka, operating as AI 182, flying from Montreal to London en route to Delhi, crashed into the Atlantic Ocean off the west coast of Ireland on June 23, 1985.
With no survivors, the crash has been described as Canada's worst terrorist attack.
Singh was joined at the memorial service by Canadian Minister for justice Peter MacKay and Ireland's Minister for Foreign Affairs Charlie Flanagan at the Irish village in County Cork.
"The creation of this pilgrimage place that I, we, can come back to year-after-year, permits me to become a mother once again...That allows me to give thanks in prayer, is sheer magic," said Padmini Turlapati, a Toronto-based paediatrician who lost his two sons in the crash.
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The bomb was placed in the suitcase of a man who had checked in his luggage, but did not board the flight.
The attack later emerged as the work of Sikh militants who wanted to strike at the Indian government's 'Operation Blue Star' at Golden Temple in Amritsar in June 1984.
Inderjit Singh Reyat, who admitted helping construct the bomb, but denied knowing its intended target, was jailed for five years in 2003.
In 2005, Ripudaman Singh Malik was acquitted of charges of financing the plot, while Ajaib Singh Bagri was acquitted in 2005 of transporting the bomb to Vancouver airport.