Translation:They did not see the stars yesterday because it was cloudy.
Usually, the negative form of the past tense is formed with níor, but feic is one of the exceptions (see also the tips and notes section in 'Verbs: Past 1').
Thus, the negative past form of feic uses ní instead of níor : Ní fhaca... In the same vein, the question past form of feic uses an instead of ar : An bhfaca...?
[I'm not a native or even a fluent Irish speaker, so feel free to correct any errors on my part.]