I'm Northern Irish.
This is a terrible idea. Northern Ireland is actually doing pretty well, and the standard and cost of living is lower than the Republic. The border is basically not there for British and Irish citizens. It's just a matter of currency change and speed limit change. The EU backing a United Ireland would cause already a lot more distrust in the EU by Unionists to the point of breaking Stormont again (Keep in mind how difficult it was for Stormont to be happy with the Brexit arrangements)
As a matter of fact, Northern Ireland is actually starting to grow. It now has an economically advantageous position that a United Ireland would destroy. It has mostly customs free access to the UK market, access to the EU single market, and now is one of the few places in the EU single market that can export to the USA with a lower tariff rate. All for the price of a what is essentially line on a map (you can't even tell exactly where it is IRL, the border isn't that strongly defined legally or geographically)
Sinn Féin are disliked by both the Irish and British governments. They are the political wing of what once was the Provisional Irish Republican Army, a terrorist organisation.
I know it doesn't curtail ideologically to either hardline side, but the current arrangement in place is good. I live in Northern Ireland and have lower living costs and more access to social services for free than someone would have in the Republic. In some cases, better than the mainland UK. We pay less for university education than England and also don't have charges for prescribed medicine or even water usage. Our police are well invested in and Belfast is now one of the safest cities in Europe if you don't get involved in paramilitary feuds or drug dealing. Life here is good, and a United Ireland would jeopardize that significantly.