When the 201s wete ordered GM strongly advised the railways to include an auxiliary engine on the loco for Head End Power (ETH) but were ignored
You appear to have missed post #2 of this thread? Also the Rolling stock section of the Enterprise Wikipedia page may help.If this really is the case isn't there calls for new trains all together or are the faulty trains relatively new?
The obvious answer would've been not only to use the Mk3 Genny Vans, but also the Mk3's themselves to boost the rolling stock fleet. The genny vans look well in Enterprise livery.
In regards line speed, it does tend to slow remarkably down when approaching Dublin. As far as I can tell this is due to IE prioritising local services over the intercity.
The bigger problem is security alerts.