Can Ireland become independent?

Should Ireland be independent? As a sub country of the isles of Britain, Ireland is one of the most unique of it's kind. Even after gaining independence from Great Britain in 1921 in the Anglo-Irish treaty. Even after the treaty was signed and finished, the new Republic of Ireland still had bitterness towards Great Britain. This was because of the treaty only gave Ireland the land that had a majority of Catholics in that region. The other faction was the protestants that still were loyal with the Crown. This caused many violent protests and battles between the two religions. Still to this day even after the U.S. got involved and tried to bring peace, the two countries Republic of Ireland and Northern Ireland still have bitterness towards each other.
Here’s the question I have, why can't the two countries become one and tolerate each others religion. In a nation like the United States, we learned how to tolerate other religions so why can't they? Please let me hear your thoughts maybe I'm wrong. I would like to know because I think the world is starting to get tolerate of other’s religions and ideas, so why can’t a small country that wishes to preserve its heritage and history become one country and unite to show the world it can happen.