Wales and Scotland, for favoring Northern Ireland. Secularists, for doing a deal with an openly religious party. Northern Irish republicans, for violating the spirit (and maybe the letter, if they don't deal down the middle) of the Good Friday Agreement and the power sharing there. It would be a disaster if mainstream republicans rearmed, although having nominally disarmed and with many of their former benefactors (Libya, USSR, etc) no longer around they might find it a bit difficult. Still, they could probably source weapons if they really wanted to, and I wouldn't be surprised if they'd squirreled away some for a rainy day, disarmament or no.

And all that is to say nothing of the DUP's rather questionable history and ties to loyalist paramilitary (read: terrorist) groups. What a crazy thing she did.