I suppose with respect to Gatwick, what I would say is that I don't think it can really handle three rotations a day, so the early morning departure only becomes feasible if one airline or the other pulls off the route.
I don't think the early morning departures issue is as much of a problem in Cork as at Shannon. Cork has about 225,000 people living within 10km of the airport. For Shannon, the figure is closer to a 10th of that. Even if you go out to 30km, you wouldn't reach the figure. The further someone has to travel, the worse an early departure is.