Mark: I would agree with much of what you say.
Dublin-Belfast through traffic is relatively low and hourly services are a pipe-dream, without the extensive capacity-enhancing measures you mention. Long-distance travel on the Cork line is far greater.
On the Belfast line, I think you mean diversion around Portadown rather than Armagh. Also there is a pretty unavoidable severe speed restriction at Drogheda, and the track curvature over a lot of the line from the border to Portadown restricts speeds quite significactly.
A proper high carbon tax should enhance the competitiveness of rail over road and also vastly enhance the attraction of electrification. After all we aim to electrify private car transport, which is quite a tall order.