There have already been road vehicles in garda markings, and buses in UTA & CDR liveries, and a double decker probably suitable for 0 scale, in CIE green. Lorries in CIE green and 1960s black cabs.
Perhaps some of the following:
1. Postal vans - to be seen on platforms loading mail off trains - older dark green "P & T" livery, or Royal Mail red for the north
2. Buses from various eras in CIE, UTA & Ulsterbus liveries.
3. Lorries with CIE (various eras), UTA & Northern Ireland Carriers liveries. The latter suitable sitting alongside a RTR Jeep, whenever that comes out!
4. ESB vehicles.