Get onto Wildwater Kayak Club and the Garda/ UCD boat (rowing) clubs. They always have people training on the Liffey and might have seen something or even taken it in. Islandbridge, where the rowing clubs are, is the last stop before town/sea. After that, I'd be taking frequent walks along Dublin Bay. I know a guy who took up paddling after he found a boat washed up on the shore. No harm in alerting anyone who is by the sea a lot, sailing clubs, John Power (canoeing and windsurfing courses), Deep Blue Sea Kayaking etc.
Best of Luck.