A Typical Hour in the Life of the Irish Border
There are 72m road vehicle crossings a year between Northern Ireland and the Republic of Ireland, according to the Northern Ireland Statistics and Research Agency (Nisra). There are also eight trains daily in either direction between Dublin and Belfast.
About 14% of those crossings are consignments of goods, some of which may cross the border several times before they reach a consumer. In border areas it is common for milk which is collected in Ireland to be pasteurised and packaged in Northern Ireland and then sent back over the border for sale in supermarkets.
Overall, 32% of Northern Ireland's exports to Ireland are classified as "food and live animals".
A no-deal Brexit would effectively end this trade. UK government advice says that "to transport animals, products of animal origin (POAO), fish, shellfish, crustaceans, germplasm or fishery products from the UK to the EU in the event of no deal, youll need to ensure the trade route for your goods allows for your consignment to be checked at a border inspection post (BIP) at the first EU country you enter for export."
https://www.theguardian.com/politics/ng-interactive/2019/sep/02/a-typical-hour-in-the-life-of-the-irish-border
The article includes charts and an animation of traffic flows based on real-world data on 26 August 2019.