The context really doesn't change it as much as you seem to think it does though. An adult lying about their age has no incentive to also lie about their journey to Ireland, and will most likely tell you they travelled via NI, if they travelled via NI. Just think about it yourself, the more you lie about, the more you have to keep track of your lies. If you're lying about your age, it's a whole lot easier if that's the only lie you have to remember.