Steve,
Last time I heard, Northern Ireland was part of the United Kingdom. I don't moan (much) when Westminster imposes its will on Cornwall.

Cornwall has yet to achieve devoved status. Like Scotland and Wales, NI is a devolved principality and Westminster has no power to interfere on devolved issues like abortion. They take the bare minimum of decisions to keep NI running until the assembly reforms.

We have a situation here in which the DUP can prevent abortion law reform by not forming an administration

The "administration" easily voted down the most limited reform in 2016, so it probably would again.
Also, it was Sinn Fein that walked out over a different issue, and neither side are "forming an administration" because it requires both.

legally I would doubt that. She's the boss of the Uk. she could enact the necessary acts to do what she bloody likes.

Not without reneging on devolution. That would not be considered acceptable by anyone.