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.

  > England has forests

Ireland had forests too. Where are they now? Mostly in English castles.

"fjords elsewhere"

Fjords in England? Where?