It’s all about risk, isn’t it?
The fact is that, under the terms of the Common Travel Area agreement, Irish and UK citizens can travel freely between the two countries without the need for a passport. That’s the case however you travel and wherever you arrive.
But the point is the “Irish and UK citizens” bit. When you arrive there’s a risk that, at the border, someone might want evidence that you’re a UK citizen and therefore have the right to travel under the CTA.
The risk of that happening is much greater at Dublin airport. Because by the time you get to the border check you’re mixed up with people arriving from places all over the world. When you arrive on a boat everyone is coming from the UK. There’s still a risk you’ll be asked to prove you’re a UK citizen, but for obvious reasons the risk is less.
And for a UK citizen I don’t think we’ve got any way of proving that other than a passport.