They definitely both happened. I'm from just before the peak in births, and there has been both significant immigration and returning emigrants.

Deregulation and promotion of competition has also taken place, and there has been a big clamp down on company and tax law, which is also important. You can afford to reduce taxes if everyone is paying their share.

Essentially, GDP is the value of all the final goods and services produced in Ireland, while GNP is the portion of income that those who live in Ireland get to keep.

For most countries, there is little difference between the two, but for small open economies like Ireland, in which a substantial fraction of the production capacity is foreign-owned, some of the income that that capacity generates leaves the country, and so there can be a larger difference between GNP and GDP.

In Ireland's case, I believe GDP is something like 10% to 15% higher than GNP.