It would need a larger city to make that happen though - Dublin is an order of magnitude smaller than London. Now a x10 scaling of Dublin would be interesting though.
England has forests, nature reserves, fjords elsewhere. That's why this region can be so dense?
Why would you want to build up everything? Where's the gain I wonder?
Setting questionable English 'fjords' aside, the point isn't that it would be desirable to have a population of 30 million in Ireland, but that it would be possible, without having the population density (and presumably poverty) of Bangladesh. You asked how a small island in the north atlantic could sustain more than 30 million people - well, South East England has a population density of about 1200 people per square mile, at a similar latitude. Transplant that same population density to Ireland and you'd fit 39 million people. So, if you want to know what an Ireland with 30 million people in it would look like, don't imagine Bangladesh, think of Southeast England.
And Southeast England is still considered a pretty beautiful, green part of the world. It contains the South Downs and New Forest national parks, the Chiltern hills, and about 60% of the land in the region is actively farmed. Of course, in the middle of it you've got London, which accounts for the overall density. So yes, a populated Ireland would be different; Dublin and Belfast would be vastly different cities; - but it wouldn't have stopped being the emerald isle if in an alternate history its population had reached 30m.
Yes it is possible, but why would you ever want that?
Why want Dublin faster grown and thus soulless?
Why want Irish spirit to be abolished and replaced with hive mentality?
I could totally understand you if Ireland would be declining, but it's thriving, attracting people, growing naturally when most nations struggle at that. You're in a perfect position and you want to replace it with something random?
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.
