Scotland is of enormous strategic importance due to its location relative to Russia's naval ports. An independent Scotland with no other backing would have minimal resources to monitor and deter Russian naval activity.
I note that if you're looking for a weakly defended EU country reachable from the North Atlantic and quietly relying on the UK defence umbrella without admitting it, Ireland is already there.
I'm also not talking about the defence of Scotland itself, I'm noting that monitoring and curtailing Russian naval activity in the Baltic Sea corridor is of wider strategic importance. If Scotland became unable to do this, Russia would have an easy exit path for naval vessels from its Baltic Sea ports.
In your example, Ireland as an EU member has significantly more access to military resources than otherwise.