How can Sinn Fein go into government in Dublin when they still refuse to call the country they'd be ruling 'Ireland'?

The coronavirus is no laughing matter, but it's only natural for people, when faced with the unknown, to make a joke of it. In recent days, as more cases of coronavirus have been confirmed, there was a picture doing the rounds on social media of a smiling man pleased that he doesn't have to worry about the virus reaching Northern Ireland because he actually lives in the "north of Ireland".