While you can find the cult favorite at local Euro-centric grocers like Cardullo’s, in Harvard Square, for the most part the bubbly elixir remains, like haggis, a uniquely Scottish obsession that hasn’t quite caught on stateside. To learn why it should, head to the Haven in Jamaica Plain, where owner Jason Waddleton laces the Day-Glo beverage with the bitter Italian digestif Fernet Branca. The result? A bracing, medicinal cordial known as the Fernet Bru.“With Irn-Bru, national identity comes into it,” Waddleton says. “Anybody who lives in Scotland aligns themselves with it. It’s like a badge of honor.”