Perhaps you mean "Admirably" - it was admirable that they had been working so hard for so long. They were not "admiringly." That sounds like the workers were full of admiration for their duties.
You are reaching the point in which you are trying to put too many thoughts into one sentence. It was 4 p.m., and the same workers had been at their tasks since the early morning. It was admirable
"Admiringly" doesn't really make sense here -- what are the workers admiring? If the speaker thinks the workers should be admired, then it would be "It was 4 p.m. and all the workers had been admirably and diligently performing their duties since the early morning hours." My preference would be to leave it out.