This is the problem with political discussion these days - nobody wants facts, they only want to hear what supports their opinion.
clearly Ireland have a big issue if there is a no deal brexit due to the big sea barrier and the UK being a land obstacle to shorten any distance. Saying that they will just sail is over simplifying the solution and impractical.
looked up an article I read on it a while ago - it’s here if you’re interested -
https://www.bbc.co.uk/news/uk-politics-44657460 (headline if not - 85% docks in UK - 60% stays in UK and other 40% onward to Europe)