Sean was last seen queuing for a Leaving Cert party at Carbon nightclub in the city.
He left the queue at roughly 11pm to head back in the direction of City Hall in Galway, having earlier been out with friends in the Hole In The Wall pub.
A Facebook page set up to find Sean is looking for volunteers to help with the search, which resumed this morning with around 100 volunteers members of the Coast Guard and gardaí from the Mill Street station.
If you have any information that can help the Gardaí to find Sean, you can contact Mill Street station at 091 538000.
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