Okay kiddo, so a long, long time ago, there was this place called Ireland. It was a really pretty island with lots of green fields and big, rocky cliffs. There were people living there called the Irish people, and they had their own way of doing things and their own leaders.
Now, one day, a group of people from another place called England came over to Ireland and started taking control of things. They were called the English and they had a powerful king who wanted more and more land for himself.
Eventually, the English king decided he wanted to be the boss of all of Ireland, so he made a new group called the Kingdom of Ireland. This meant that Ireland would be ruled by the English king and his people instead of by the Irish leaders.
The Kingdom of Ireland lasted for a long time, and it was a complicated time with lots of fighting and disagreements between the English and Irish people. But eventually, things changed and Ireland became its own country again with its own leaders.
So, the Kingdom of Ireland was a period in history when Ireland was ruled by the English king and his people, and it was a time of change and conflict for the Irish people.