I can't help you much with the marques but regarding the haulage firms active in the 1960s, I think it was still restricted in terms of carriage of goods for hire up to the 1970s. You would still have options for adding lorries though. Businesses would have had their own fleets for the haulage/delivery of their own products - including the milling companies. Farmers would have made extensive use of their own machinery (tractors and trailers) to transport grain, and there was/is also a system of farmers' co-ops with their own lorries.
In terms of local traffic on nearby roads, Irish Sugar who had a plant in Mallow would have had their own fleet too and there's probably good archives somewhere for them and other semi-states.
Also, in rural Ireland, lorry's from businesses like Kelly's Foundry in Portlaoise (picture below) could be found delivering hay sheds everywhere, and its also plausible that a mill would need steel structures? I believe the picture of Kelly's fleet below is 1950s or early 1960s so I hope its helpful in terms of the kinds of vehicles on the roads in Ireland then. (Portlaoise is my hometown and despite it being long gone now, Kellys is still very fondly remembered in the town. Not just a midlands business, Kellys erected sheds all over the country)
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