This is not about Joe Biden's Irish background. Outside Bob Jones University, you would never meet an American who had any sympathy with the Unionist position in relation to Northern Ireland, or who regarded it as anything other than baffling. You just wouldn't.
The Scots-Irish were particularly ferocious fighters for independence from Britain, while the WASPs are unique among American ethnic groups in being defined by having fought two wars against their ancestral homeland in order to secure liberty from it. They are in any case outnumbered by the Irish Catholics who have long been the richest ethnic group, and they are dwarfed by the Germans, who these days are dwarfed by the Latinos.
Come to that, for about as long as the nineteenth-century Protestant fundamentalism of the DUP has existed, then its adherents in Britain have been outnumbered by Irish Catholics, who have long been far closer to the economic, social, cultural and political mainstream. On this side of the Irish Sea, people with strongly Nationalist views on Ireland, while not necessarily numerous, vastly outnumber people with strongly Unionist views on Northern Ireland, who barely exist.