In the period after the Good Friday Agreement the DUP absorbed a considerable element of the more hardline wing of the UUP, so they became broader based than they had been; indeed Foster herself is an ex-UUP Anglican from County Fermanagh. So in practice the DUP is a Protestant identity party.
That said, there's definitely a "Bible Belt" vibe in parts of NI, especially in County Antrim, and that goes beyond Paisley's church and would have provided a lot of the DUP's original base.
Ironically, the antecedent of the modern DUP got its feet wet in Terence O'Neill's old Stormont constituency, Bannside, which Paisley won after 1969. Around then it was a mix of new middle class and partially rural working class, but mostly all Protestant. Just by Paisley's vote share there in 1969 and after, he drew across class lines, which speaks to the malleable cultural/identarian appeal of the DUP--their most successful (and most depressing, imo) campaign tool ever since. So, for now, I won't underestimate Poots, especially if he does leave the FM job to another.
