8.50 is usually NIR CAF, 9.50 and 10.50 are usually loco hauled, 11.50 ICR.
Hourly service is really too much to ask from the locos and the DDs, the infrastructure of loco-hauled stock isn't in place anymore. There's no loco to save a train if it sits down say in Dundalk or somewhere nearby, the CAFs aren't designed for such a service. Only the ICRs are a decent substitute, and there's not enough of them to fully substitute.