So the NI 'curfew' seems to be covered by the regulation below - a power to direct a person home - but they have to be engaged in, or intending to engage in, a prohibited activity (essentially a gathering) and the exercise of the power must be necessary and proportionate.

A separate regulation covers exercise with members of your own household which also seems to be allowed after 8pm.

There is no general curfew, but a power for a relevant person (generally police), to send someone home who is engaging in, or intending to engage in, a prohibited activity. There's also a necessary and proportionate test. This seems reasonable given nothing is open after 8pm.

Walking and exercising within a household after 8pm is allowed. The 'curfew' only applies if you are travelling to something you shouldn't be doing anyway and the 'punishment' is being sent home.