I grew up on the border in N Ireland and I can tell you from personal experience that there are so many fields and roads (and even some houses) that have the border running through them that people could easily use these to cross rather than any official checkpoints. It's always been that way ever since Northern Ireland was created and the island was partitioned. The border has been taken advantage of, and ignored, by terrorists and smugglers and locals for both practical and political reasons.
Even during the Troubles, when the British army and HMRC tried to lock it down as tightly as could be managed by declaring checkpoints to be the only official ways to cross and that minor roads were unapproved, and in some cases blown up to make crossing difficult, the locals ignored it and even filled in the bomb craters to make the roads passable again. Huge numbers of people and livestock living in the area (as well as goods, tourists, workers, and people from other parts of the island) have crossed over it countless times every day for decades.
New rules since Brexit have made it irritating in ways small and big so that, for example, people living on the border now need two mobile phone contracts (one for UK and one for ROI) because mobile phone signals ignore borders and you could be living in NI but only be able to get ROI phone signal which would cost a small fortune in roaming charges if you didn't have an additional ROI or UK mobile contract. Trying to send gifts and post to family members on the other side of the border has become a pain in the arse with new customs regulations. There's been an increase in cultural tensions for people who live on the border because being in the EU helped to mitigate the fact (for Nationalists) that Ireland was partitioned and that NI was in the UK - with both states being in the EU the border issue could be ignored and Ireland didn't feel as partitioned as it had been. Now, for some people the changes arising from the UK leaving the EU have brought the spectre of violent disagreements about partition back into the worries of daily life and have upset decades of cross community peace building.
The best (and easiest) system for everybody in Ireland was the open border that came with being in the EU - it made life easiest for everyone living on, and working across, the border itself and for those with family on the other side of the border. It was great for businesses large and small as it made it easier for say a dairy company to buy milk from farmers on both sides of the border on a daily basis and combine it into products that were sold all over the island. It helped to diffuse sectarian tension over partition and it improved the economy of border areas which, pre-EU, had been hugely disadvantaged economically. For the majority of people living in communities on the border in Ireland, partition of the island is viewed as being an imposition by the British State and the Common Travel Area didn't achieve much in the way of positives for the people living on the border. It was both ROI and UK being in the EU that paved the way for the daily improvements in quality of life and political sentiment. Now the new rules introduced post Brexit bring frictions back into daily life for people and it's difficult as well as unpleasant to be going backwards in this way. And I wish all of these things, and more, had been taken seriously by successive British governments during the Brexit referendum and negotiations.