The capacity of a DART car is approx 50 seated and 150 standing, so an 8-car unit could potentially carry 1600 passenger. If IE ever achieve 10 minute frequency, the line capacity would be 9600 per direction per hour.
The original 30m Luas had a capacity of 256, and the current 40m units have a capacity of 358. I can't find an official capacity for the new 55m units, but a back of the envelope calculation suggests it would be close to 500. I also haven't seen capacity figures for Metro, but I believe it's planned 70m trams would be used, so I would guess these would have a capacity of around 650. At 3 minute frequencies, Metro should have a capacity of 13,000 per direction per hour. You would need to get the DART up to 8 trains per hour to come close to that capacity.