Why would anyone ever want that? Because people have different opinions about what is best. You are absolutely entitled to think that a more populous Ireland would be a bad thing, but you're being downvoted because you can't seem to see that that's just a personal preference, not an absolute truth.

Personally, I live in New Zealand (population: 4.5 million), and I'd strongly prefer it if we had twice as many people. A bigger population can sustain a healthier economy, with more opportunities for everyone. However, I'm well aware that I'm an outlier; most people think differently and I respect that.