The answer about the GF is it would have been locked by a key on the staff. With all signals being retained, I would expect that the home and starter levers must be capable of being left reversed at the one time, much as they are at Portrush to this day (and will continue to be when Portrush ceases to be a Block Post later this year.)
It's quite probable that a large type ETS would have been retained in the station building, but the instrument only used as required for a second train to come down the branch.
In that case, the scenario you would imagine would be:
1. Train arrives in platform, runs round, shunts out beyond home signal, and reverses into siding, where it is locked in via the GF. ETS can be replaced in instrument and another one obtained.
2. Next train arrives, runs round, shunts into loop
3. First train shunts into platform and departs with passengers
4. Staff obtained by PIC to release GF and shunt second train either back into platform or into siding, assuming that shunting
Of course, if second trains were exceptional, then the instrument would have remained like Waterford Abbey Junction-New Ross and Queenstown Junction (as the staffs remained despite Queenstown having become Cobh in the 1920s)-Midleton, both of which had large ETS instruments and the hypothetical ability to revert to ETS until the 1990s in the case of New Ross, and 2009 in the case of Midleton, but in practice neither had done so in probably decades.