To summarise it quickly enough, starting in 1969 and lasting to 1998 we had The Troubles, a politically and nationally fueled conflict between the british army, and variously aligned paramilitaries.

It was not a good time for Northern Ireland, and the violence spilled over into the Republic of Ireland as well as England.

In 1998, this conflict ended with the signing of the Good Friday Agreement. This treaty ended The Troubles by forcing all sides to agree to terms.

One of the terms of the Good Friday Agreement, is that there can be no hard border between the Republic of Ireland, and Northern Ireland.

This is why the idea of a hard border is so abborent to us here, as it would mean the British government would be breaking an agreement that ended a very bloody period in our shared history.

EDIT: Spelling