Stray away from red hair. Most people in Ireland have dark brown hair. Blue eyes or brown eyes as well. Small, round faces. Mesomorphic body-structures. Pale skin. Can either have a lot of freckles or a metric shitload of freckles, though not necessarily on the face, the arms and legs being quite common places as well.
And an odd little fact: it's been observed on both Irish people and Scots that the second-toe is as long as the second. This is called Celtic toe. I have no idea if this is as in-depth as you want, but I just find that fascinating.