The specialization of the cocktail world means that, for aficionados, it’s no longer sufficient to pop into whatever bar you pass and hope they’ll know how to make your hand-selected house barrel five-rum-blend tiki flip. Knowing ahead of time who’s behind the bar on a given night can help bar-hoppers plan more efficiently. A new free application (for iPhone and Android systems) designed by developers TJ Connelly, of Cambridge, and Ian Stanczyk, of the North End, called On the Bar, helps cocktail fans connect with their favorite bartenders. We asked the duo to explain how it works.