Many would sacrifice a point of GDP to get back that sense of national community. European people have been robbed of their identity and their roots have been taken from them and are vilified for speaking about it. Some sort of restoration of this loss would already lead to greater satisfaction and happiness for many, rather than any economic bonus, which would be a reality anyway. I wouldn't look at it merely from an economic perspective.
Solution-wise, there are many ways to go about it. Some examples:
Countries have to shut the gates and first take care of the asylum applicants and illegal immigrants that are already requiring attention. Illegal immigrants have committed a crime so their sending back is imperative. Asylum, according to international law, must only be accepted by the first safe and stable neighbouring country a refugee enters. Any asylum granted by another country is charity and goodwill. This is done on a case-by-case basis.
Foreign nationals or dual-citizens who have migrated to the country as an adult and have committed a capital offense should be repatriated without question. The severity of the crime in relation to whether or not someone should be deported is up for debate. The country of origin should be held responsible for the criminal in case of statelessness. Any dual-citizen who wants to hold public office in their host country should revoke their foreign citizenship(s).
A hard pushback policy would have to be implemented to deincentivize people from migrating and spending their money on human traffickers, risking their lives to cross desert or sea. Human traffickers should be cracked down hard, which is very unrealistic of course because of their ties to intelligence, NGO's and financial racketeers. Refugees can be helped better in their countries of origin or safe neighbouring countries, without being uprooted from their cultures and people, for ten times less costs. Refugees could acquire asylum given they will return after the situation in their country of origin has stabilized. Second-, third-, fourth- etc generation immigrants that have problems integrating into larger society should be helped, if willing, to find their place in their country of origin instead of infinitely consuming welfare resources.
Making babies should be incentivized as in countries like Russia and Hungary. In Hungary, for example, tax rate declines until a woman's 4th child. Every woman (of Hungarian descent) with 4 or more children doesn't have to pay income tax. Obviously promotion of marriage (capable of procreating) should be preferred over the promotion of divorce and sterile relationships.
In your case
@justjess, or your Irish husband, or any Catholics, there's not really an issue. The US was of a different make up from the start than the average European nation, with Irish, German, English, Natives, African, etc. The large majority however (90% in the 60s) was of European descent and Christian, so you both are part of that larger ethnic fabric. I think efforts should be made to preserve whatever's left of that if you want to continue to live in the America you've come to know.