It's worth noting that the UUP have traditionally been affiliated with the Conservatives and even took the Tory whip at various times, although there were also various instances of them splitting off in protest. The DUP don't have that history. When I was last in Northern Ireland the UUP would have been very much more likely to enter into an arrangement with the Tories, while the DUP were significantly more hardline. As the anonymous commenter above suggests, that may well have changed, particularly with the DUP having spent some years in devolved government.