"It is not a good night."
Translation:Chan eil deagh oidhche ann.
Correct - chan eil oidhche math describes a night as being not good, in other words the opposite of tha oidhche mhath.
In this case, we are describing a particular night as not a good night, so we need the negative of tha oidhche mhath ann, which is chan eil oidhche mhath ann.
You could also define it as not a good night by using the negative of 'S e oidhche mhath a th' ann, which is chan e oidhche mhath a th' ann.