Sorry but this proposal is to say the least totally unrealistic. For one thing motorways are built with gradients which are much more severe than railways, and they are often quite curved by railway standards.
It Tipperary CO Council want a motorway to replace the N24, then this would kill off the railway for sure. I suppose the CO Council would lobby for both railway and motorway given that taxpayers elsewhere would largely be the payers.
A serious upgrade of the line would cost a lot less than any motorway proposal, and could result in genuinely competitive journey times, but that would be totally counter to NTA and Dept of Transport thinking.