I used to hate it, but it actually makes more sense when you're actually in/moving around Dublin lol.
Most buildings in Dublin are less than 5 storeys high, so you can see the Spire from most of the city centre, and get to the rest of the city from it fairly easily.
The Spire is designed so it works as a landmark you can see from almost anywhere.
Before Google Maps was really a thing, and even now it is, you could use it as a physical reference point to move around the city, and you're kind of drawn to it one way or the other naturally. It's kind of a giant IRL map pin.
That means there's always a group of people at the base; teenagers meeting up with their buddies before setting off for the day, tour groups, folks from out of town just off the bus waiting their friends, and - my perpetual favourite - well dressed people with visible nerves trying to act cool as they wait for their dates.
So rather than a blimp defense system, think of it more like a spawn point.
Also when there's a Star Wars movie they turn it into a lightsaber.