The two Donegal narrow-gauge railways both moved over to run replacing bus services, in different ways. Because of their cross-border ownership they didn't get included in either side's rail or bus consolidation, but remained independent.
The County Donegal invested in some fairly basic diesel railcars, but gave up rail services in 1960 for bus operation, which nevertheless was provided under subcontract by CIE with their buses. These still got based at the old railway stations, and the bus garages were on old railway property. This arrangement lasted into sometime in the 1970s when it was fully transferred. As the buses had always been CIE ones, there wasn't much change to notice.
The Lough Swilly provided their own buses, and had pretty much changed over by the 1940s, although like the County Donegal there would be special passenger trains provided for special large events, which presumably people just got to know about. They had an extraordinary ramshackle bus fleet, the sort that never buys a new one, and again were based in former railway premises. This lasted till very recent times, when they finally went out of business and the Irish state operator took over.
We did visit Donegal about 10 years ago and saw all this. The elderly and (alas) unwashed Lough Swilly buses, then trading as "Swilly", were still running out of Londonderry, sadly almost empty, straight across the border, and although almost all their mileage was in the Republic they had Northern Ireland numberplates. In Donegal we coincided with some sort of fair day. We managed to see three former County Donegal locomotives, one in the museum in Belfast, another sat on the quayside in Londonderry, and a third on a plinth at Donegal station. Looking at Google maps, these last two have moved on.