I also just think that the combination of Guiness and Rosemary to me equals a sort of soapy metalic taste?
Interesting to hear how it turns out!
I've made it without the rosemary in the past and I find it tastes better with it. I add a little bit of brown sugar to reduce some the bitterness of the Guinness. It's been an awful winter so I've been making it just about every week! I usually eat it with some fresh sourdough bread from the Italian bakery next door.