RootsChat.Com

What political and economic advantages are being referred to?

Maybe I have misinterpreted. Perhaps it's the church that was said to enjoy those advantages, rather than individuals who profess to be Church of Ireland?

Probably a bit of both - The Church had a lot of power and being on the right side would provide some advantages and connections.
Presbyterians tended to be descended from Scottish incomers and often found them selves stuck between two warring factions and trusted by neither. In 1639 The English Authorities tried to impose the so called Black Oath on all Presbyterians (basically denying the National Covenant in Scotland and accepting Charles II as the Head of the Church). Many fled back to Scotland and others started the emigration of Ulster Scots to the Americas. The authorities started systematically disarming all Presbyterians (fearing an uprising) and when the local Irish did rebel in 1641 many were killed in the sectarian violence. (The numbers are often disputed and seem to depend upon which side is writing the narrative)

In the USA it would appear to depend on which state you live in or get married in but according to this article from Wikipedia https://en.wikipedia.org/wiki/Cousin_marriage_law_in_the_United_States  their primary concern is in regard to first cousins.

As to whether there was any benefit for those involved in being parsimonious with the truth it wouldn't be the first or only time its happened.

Mental Deficiency  Act 1913

Reading the wording of this legislation shows it's potential for exploitation

https://education-uk.org/documents/acts/1913-mental-deficiency-act.html

For quite some time Electricity was considered a cure all for many afflictions and although many practices had stopped by the early 20th Century others were still in the development stage (electro convulsive therapy)

With your diagram (chart) on screen right click on the individual you want to make changes to which gives you a drop down menu.

from this menu select "Format" which give you the screen attached which will enable you to override the standard/default colours fonts etc.