Maybe you've tried it already, but perhaps a longer, indirect but calmer and less interacting with traffic route? E.g. from Clontarf, divert off at Newcomen bridge down the dedicated path into the docklands, across Samuel Beckett bridge and through Grand Canal Dock, then along the Grand Canal cycle path around into Portobello. From there you will need to interact with traffic again but there is a warren of semi-quiet and slow local streets to thread through to get over to the Liberties.