Witness statements from the crew of LÉ Niamh, along with photographic evidence of the capsize of a grossly-overload fishing vessel, were provided to the Italian authorities.
Lieutenant Commander Daniel Wall travelled in December to a court sitting in Sicily to provide evidence against five men charged with aiding and abetting murder.
He was commander on August 5, 2015, when the ship assisted nearly 600 migrants, packed into a vessel off Libya. The boat capsized just before the ship reached it.
Hundreds were thrown into the sea. Migrants in the hull were unable to escape. Most had no life jackets.
The LÉ Niamh crew threw lifejackets and miraculously saved 367. They recovered the bodies of 25 but more than 200 drowned without their bodies being found. Survivors identified the suspected people-smugglers. They were segregated and handed over to authorities in Palermo.