UMass, quoting Roger Yepsen, calls Shamrock "the East-Coast's version of the Granny Smith."
It is medium-sized and, as the name suggests, a bright Kelly green. The fruit I am tasting has a small and faint red blush and also some odd black stippling over part of the skin--some kind of russet I imagine. There are dark green spots that are light colored in the blush.
The flesh is light white-green, crisp, and juicy. The skin is on the thick side. The "Granny Smith" comparison is just what ad executives call positioning; Shamrock's flavor is both sweeter and less tart than a Granny Smith, with a little spice and berry notes. There is some acidity, not overwhelming but enough to balance the sweetness for a lively and refreshing effect.
Some residual astringency.
I've been eating a lot of Macouns this week, but yummy as those are it's nice to take a break and Shamrock is a worthy change of pace.
Note: I had another taste of Shamrock in the summer of 2010.