In my own family, my father is one of six boys and one sister. His father was one of four boys with two sisters.
Based on the photographs I've seen of them over the years, none of those ten males have a particularly strong resemblance, although they're clearly related. Some were blonds, some brunets, some blue-eyed, some hazel, some brown.
In my mother's family, she is one of three girls and two boys. Both of my maternal uncles have had a strong resemblance to each other and the same for my brother, as those three do and did with their father and their uncles.
All my male cousins on that side of the family also have a strong resemblance -- the same coloring, the same build, the same shape of the head with similar faces. Every single one has or had dark hair and brown eyes with an olive complexion, the same Irish coloring as Colin Farrell. The females are all over the place with coloring and facial structure.
Looking at photos of my husband's family, they are like the males in my mother's family -- all variations on the same dark Irish male archetype. All have dark hair, dark eyes, olive skin. The women, however, are not nearly as similar beyond superficial resemblances -- some are natural blondes, some brunettes, some redheads. Eye color varies among the women, too.
And, of course, our two boys have this same dark coloring and very similar faces. Even though they're twenty-eight months apart, people will often comment how much they resemble each other. So I wonder why this so-called Black Irish resemblance is strong in males but weak in females in two unrelated families.
If we have another child -- and we're still trying regularly, thank you -- it will be interesting to see if he (or she) keeps up this tradition. I would imagine a boy would be another dark Irish fellow, while a daughter could be anything.