This sounds interesting. Have not seen Mirto over here so might take some online investigation. Also no directions but I'm assuming shaken over rocks given the amounts, correct?
Mirto is a Sardinian liqueur based on myrtle berries. It's sort of bitter-sour and resinous, and in a pinch, I'd try 1/2 black currant liqueur, 1/4 gin, and 1/4 dry red wine to get close.