I mean they're not really wrong. If the UK can't refuse migrants arriving by boat on the basis of them coming from France, which is a safe country, then it stands to reason that Ireland can't refuse ones coming from the UK. Thinking one of those is fine but the other isn't is certainly hypocritical.
That said, I do find the political football of boat crossings to be quite tedious. They make up a tiny portion of immigration into the UK, yet it's all the government and media seem to talk about.