Birth control definitely plays a role. But we also tend to think that sex before marriage was very rare in earlier generations when it was actually quite common. There were huge percentages of pregnant brides throughout history.On the bold I'm sure that is not true in comparison. It probably has more to do with the ability to use birth control and have an abortion.
The DNA tests are more accurate than guessing but they're not anywhere close to 100%. My BIL did the Ancestry kit. It showed a high percentage of Iberian Peninsula but no French, French Canadian, and nothing from Ireland/Britain. The family has records from France and they left Nova Scotia and moved to Louisiana during the Expulsion. That's their Dad's side. Their mom's family came over from Ireland in the early 1900's. She knew her grandparents and they were definitely Irish. Maybe they mixed up his sample.
The best way to tell would be if there were any relative matches. And, if not to have a sibling complete a test. If he matches to someone then the sample was certainly not swapped.
There could be explanations for why the results came back unexpectedly. Perhaps someone along the way was adopted (this seems to be the most common explanation for “wrong” DNA results). Perhaps the grandmother had an affair. Perhaps although the grandmother came from Ireland at least 50% of her DNA was non-Irish. (You only inherit 50% from each parent so there can be things that are present in the parents but not in their children).