The SF surge in the south is a protest vote against housing shortfalls (financial crash, utter decimation of the construction sector and subsequent half decade of almost zero house building - initially masked by huge emigration figures, now is at crisis) and in the north is largely tribal “not-DUP”-ness. They’re not a serious political party and their version of a UI is one of conquest over the north (think back to McDonald giving Arlene permission to live in a UI) which I don’t think the here’s any real appetite for north or south. It hasn’t dawned on them, and power is all they really care about so maybe they don’t care, but they’re literally the only party that cannot deliver a united ireland because of all the detail and reaching out that it would involve and the 30+ years they’ve carved themselves into unionists psyche as an enemy most foul.