Similarly on the IMR the crossings at Ballasalla and Port St Mary are half barriers although these were both gated crossings within my memory span. I think I'm correct in saying there were colour light signals at the platform ends at Douglas, but these have reverted to semaphores.
You are correct that there used to be two colour-light starting signals at Douglas. They were replaced by a bracket with motor-worked semaphores, recovered from the Ffestiniog's aborted TanyGrisiau signalling scheme.
Are you sure about the crossings? All the barrier crossings on the IMR I can recall are FULL barrier crossings, not half barriers.
When the IMR was rebuilt in conjunction with the water main scheme, all the old gated crossings were renewed as automatic barrier crossings. Some have four barriers and some just two, depending on the road width, but in all cases they are full barrier crossings that fully close off the road.
There are significant differences to UK practice. For example, there are no track-circuits, just one treadle on each side of the crossing. When a train approaches the crossing and operates the treadle on that side then the warnings will start and the barriers lower. The treadles are sited sufficiently far from the crossing to ensure that adequate warning is given to road users. The crossing will then stay closed after the train has passed over the crossing until the train reaches the treadle on the other side. So a crossing can stay closed for a noticeable time after the passage of a train - this is especially noticeable with short trains. However, there is no danger of cars weaving the barriers after the train, as they are full barriers.
The crossing at Castletown is particularly odd, as on one side it has flashing yellow lights, not flashing red. The railway here runs parallel with and immediately adjacent to a main road. There is a T-junction off this road, that immediately passes over the railway. You wouldn't want a car to stop at the crossing itself, as it would block the road junction. Equally you wouldn't want flashing reds on the main road, as you wouldn't want to stop all the traffic on the main road just because the side-road was closed. So they have provided flashing yellows, like the flashing yellows that you get for school crossings.