system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is Ireland divided into two (Northern Ireland and Ireland) and is there a chance that they'll ever be combined back into a single Ireland?
gpt
The entire island of Ireland was once a separate country, but became part of the United Kingdom in 1801. It was an unhappy relationship, and Britain's treatment of Ireland during the infamous Great Famine of 1845-1851 caused a great deal of resentment. Movements sprang up campaigning for Ireland to regain its independence, mostly among the majority Catholic population. Independence was less popular among the minority Protestant population, descended from immigrants from Scotland and so concentrated mainly in the north of the country.

In 1916, things got violent: an armed insurrection by a relatively small group (the Easter Rising) was met with brutal violence from the British, which only made the Republican cause more popular among the Irish. The violence escalated into full-blown civil war.

In 1921, a treaty was signed giving Ireland almost complete independence, but with a clause allowing Northern Ireland to remain part of the UK. Northern Ireland, which has roughly half-and-half Loyalist (Protestant) and Republican (Catholic) immediately invoked that clause, and so Ireland was split.

There followed several decades of terrorism in Northern Ireland, as Loyalists and Republicans battled it out on the streets, and the British Army was deployed to keep the peace -- not very successfully, and it was often accused of being anything but impartial (in one notorious event, the Bogside Massacre -- popularly known as "Bloody Sunday" and the subject of a U2 song -- 26 unarmed civilian protestors were shot by British soldiers, 14 of them fatally).

The violence, for the most part, was finally ended about ten years ago (the Army withdrew in 2007), and the province is now governed by a power-sharing agreement, with two leaders: one Republican and one Loyalist.

Most civilians would probably tell you that they don't mind whether the province remains British or is reunited with Ireland, just so long as there's no repeat of the endless violence of the 20th century.

Because of various agreements and treaties between Ireland and the UK, the border is not a very big problem at the moment. Both countries are part of a Common Travel Area, which guarantees freedome of movement (the CTA includes a few territories that are not part of the European Union, which is one reason Britain and Ireland opted out of the Schengen Agreement which abolished routine passport controls between most member states -- it would have conflicted with the CTA).

The problem now is Brexit. Northern Ireland actually voted Remain, and this has prompted some people to suggest that now is the time for Northern Ireland to leave the UK and rejoin Ireland. Brexit will mean that the border between Northern Ireland and Ireland forms part of the border with the EU, and it's not known how that will affect trade and the CTA.