The problem you have, is in essence they have ceased to be the Conservative and Unionist party for a long time. The argument is too difficult to make and provides very little capital/gains in key areas now. It's not a vote winner. And that is what matters to a political party, who increasingly will bend every rule, seemingly break multiple rules, lie to Parliament, lie to the monarch etc in order to maintain their own power base.
In Scotland they have stoked the fires of independence. In 2014 they promised them a whole range of proposals and guarantees if they stayed. They were ignored. They then ignored the will of the 2015 election in Scotland (which as explicitly anti-austerity) and ignored the vote in Scotland (where every single constituency voted to remain). What you have is a cabal of largely southern English Mp's who believe the myth that they create all the wealth so only their view matters.
It comes as no surprise NI will be sold out. The preservation of their rule, if only for another few months will trump the sacrifices made by the unionist community in Ireland every day of the week.
I don't think they stopped being unionists - I just think they got so incredibly arrogant and dismissive of certain areas of the United Kingdom that they think they can do what they want and run roughshod over people and expect them to just bend over and take it.
I honestly don't think they see the danger to the union at all. They're blinded by ego and greed.