These top 2 pictures show how I first did them. I thought a star tip would work great for the "foam" at the top. Well, I didn't love the way they turned out and this way used A LOT of icing. Near the end, I was close to praying to St. Patrick to let the icing hold out through the last cookie.
So, here's what I finally came up with. The foam part is just filled in as normal with flood icing and after it dried, I piped little curlicues for the bubbles. I like this much better. 
Happy green beer drinking and eating!