Ireland was a net exporter of people for about one hundred years. Millions of Irish people left, all young. Millions of women in particular left the west, and communities were devastated. The effects of this were still to be seen in small communities in the west and northwest as late as the 2000s, with a much higher level than normal of schizophrenia, particularly among elderly bachelors living alone or with their bachelor brothers in run down cottages without running water or indoor toilets (women tend to be faster to adopt domestic improvements and to nag men to get improvements done).
At the same time, families were able to use a relative in Oz or the US as a base to send others abroad temporarily or in lieu of enduring poverty and lack of opportunity to go further via education at home, and this was particularly useful when times were rough in Ireland and for families for whom third level education was not going to be a possibility. Emigration from some parts of the west was often seen as a temporary measure, with families coming and going from Boston or New York to towns and farms in the poorest counties. I knew many schoolfriends who were entitled to American citizenship by virtue of a grandparent born in Philadelphia and able to prove it. They knew if they got a degree or some training they could hop on a plane and settle in the US whenever they wanted, with little or no red tape. Irish people could serve in the US military and enjoy VA educational benefits, healthcare and pensions.
And of course money flowed back home that benefitted the local economy greatly. Farmers stayed on farms because they could afford stock or seed or machinery. Shopkeepers stayed in business when money from a rellie tided them over when locals had to ask for credit beyond the normal limits. Bank loans were paid back. The experience of life in the US served as an impetus towards modernisation even in the most backward of places. People liked the nice bathrooms they had in Boston. They liked the flower gardens and the decent public libraries. When they returned they wanted the same in Mayo.
It was and is very much a plus for Irish people to have links to the US and it must be said, for the US to have had a constant stream of immigrants from Ireland to build railroads, canals, cities, sewer systems -- just as they did in Britain too. Ditto for the chance the EU offers to all its members, and the benefits flow both ways. Don't forget in these days of (misguided imo) belt tightening that British citizens can go all over the EU and that educational opportunities in particular are available on the same basis as they are to locals.
The downside for Ireland was that as long as the pressure valve existed, political pressure to improve the economy was relieved. What made a difference for Ireland was membership of the EEC (which meant many elements of Irish life had to be raised to first world standards) and the economic growth thanks to massive expansion of educational opportunities, a change of course on the part of the Department of Finance to favour encouragement of growth, and the sudden availability of markets for Irish products (EEC/EU) that happened from the 60s on. Ireland eventually became a net importer of immigrants and probably will again.
Romania, etc. (if we are again talking about mass immigration of Romanians) may follow the same patters as Irish emigration did -- people will go where they are already established and will come and go, will send money home that will eventually grow Romania as well as the places they settle.