Add flour, baking powder and sea salt to a large bowl, whisking to combine and aerate. In a medium bowl, combine buttermilk, butter, eggs, and egg yolk, mixing until just combined. Pour the wet ingredients into the bowl with the dry ingredients, and stir until just combined. Don’t overmix here. It’s ok to have lumps, but make sure any large patches of flour are mixed in. Stir in Dubliner cheese and rosemary, then let the batter sit at room temperature for 30 minutes.