The sirens mentioned are two different ones.
The fire station one was used to alert off duty fireman that they were needed to man the second appliance, should they be out of the homes, after an initial call out for the on duty crew. A bell was installed in the homes of all firemen who were available for a call out. The bell arrangement was used at night of course so as not to awaken the whole town, but it was known for mistakes to be made, much to the annoyance of local residents.
The fire brigade use of the siren ceased when a pager system was introduced however it was still maintained as part of the civil defence system and I think is still in place.
The shipyard had a "hooter" that sounded at 7.25 and 7.30 each morning, and 12.55 and 1.00 in the afternoon as a warning that shifts were about to start. If you failed to be in the gate by the time of the second warning you were locked out and lost time. The time keeper was Mr Farley who enjoyed closing the gate on the stragglers.