Why was ireland split into two parts

Ninety years ago Ireland was split in two after people living there went to war against their British rulers. The south became a separate state, now called the Republic of Ireland. But the break-up led to decades of unrest and violence in Northern Ireland, which remained part of the UK.

Ninety years ago Ireland was split into two parts ok I'm in sixth grade

It holds it together more firmly so you can firmly grasp it!