I added one small thing to the shamrock from the honeymoon t-shirt, but I think it really looks better. So I made a shape that follows the edge of one leaf, but a little smaller. I filled it with a greyscale gradient, and then set the transparency to "screen" mode.
I also made a small change to the back of the shirt. I added a picture of a typical Ireland road sign and just moved some of the other images around. The road sign was made by using the live trace feature on an image from the internet. Again, a small change, but I think it really improves the overall look. Too bad I made these changes after the shirts have shipped.