Rather than put in a hard border at the Ireland/Northern Ireland frontier, which many on both sides think might lead to a resumption of The Troubles, the Irish Backstop would leave Northern Ireland in the EU customs union and put the customs enforcement between them and the Island of Britain proper.
Complicating things are the Democratic Unionist Party, Northern Ireland Protestants who May needs in her governing coalition in order to keep the majority. They're opposed to a hard border, but also to the Irish backstop.
In April 2018 I took a train from Belfast IN northern Ireland (UK) to Dublin in the “Republic of Ireland” there was no border as far as even having to show my passport...
The only difference was the use of Euros instead of English Pound Sterling because the Republic of Ireland is part of the EU...
Yeah, that whole thing turns out to be very complicated. A majority of the Northern Irish voters voted to remain in the EU, but they really don't want to face a vote that might be in favor of pulling them out of the UK. Certain Republic politicians see it as an opportunity to reunite Ireland altogether. This is their 13th year without the checkpoints that were such bomb magnets during The Troubles, and nobody really wants to start that up again, or almost nobody. What certainly did just blow up was May's coalition.