Unfortunately, the CSO don't provide data for internal flights.
However, remember that 20% of a Ryanair aircraft is still nearly 40 people. If that's a bad flight, an operator of ATR 72s or Q400s should be able to do something.
Waterford isn't as close to another airport (nearest is Cork 134km away) as Galway (Shannon at 79km and Knock at 81km). Also it's not as runway constrained. Finally, although Waterford is a smaller city than Galway, more generally the South-East is more densely populated than the West.