I get the importance of DU and personally think it is more important than Metrolink and should have been prioritised. But given that policy is the Metro comes first and the earliest we are likely to see DU is around 2040 would quad tracking between Connolly and Tara now not at least alleviate that bottle neck and improve capacity significantly? If trains cant pass each other out it is difficult to see any further improvements for IE in that stretch of the city centre over the next 20 years, possibly longer.
If you mean faster journey times, then no, I don't see any major improvements in that area without significant investment in quad tracking, passing loops, etc. and even then there is only so much you can do before you run into platform/station capacity constraints.
If you mean improvements in capacity? Then yes, more can be done with the existing network.
More DART carriages, so all DART trains are 8 carriage trains would go a long way to helping ease the overcrowding we are seeing.
More commuter carriages and trains are badly needed.
Commuter trains being hybird eletric-diesel, so that they can act more like DARTs along the electrified sections of track, stopping more frequently at stations, would help better balance capacity between services (e.g. where people are seeing half empty commuter trains pass by jam packed DARTs).
There are things to be done infrastructure wise that can help. Closing level crossings would be a major one. But to be honest I wouldn't expect big journey time improvements from them, I suspect time gains will get eaten up by increased capacity, more trains, more frequency, more stations, more stopping.
Honestly I'd say the best to hope for is journey times remain relatively static, while capacity grows greatly.
I'm looking forward to see what they come up with for the DART expansion plan.