The Irish Divide

I am very interested in learning more about the reasons for the Northern Ireland divide. I am curious about why a country as small as Ireland would not be united. I am oblivious to the history and political consequences that caused this tension; however, I am interested in learning this information. The difference in currency, culture, religion, and government between the two regions is quite striking. Despite their shared heritage, I am unsure of what connections the North Irish and South Irish people have.