It has United States preclearance and quite a few destinations to cities that they don't fly to such as San Francisco.
Dublin has pre-clearance advantage for now, but what DUB has over MAN is not so much 'destinations', but, frequency.
Destination wise, what DUB has that MAN doesn't are Charlotte and San Fransisco. On the flip side, MAN has Sanford, Las Vegas and Miami that DUB hasn't, so one could say MAN has the edge on that front.
The frequency from DUB however, is much higher on most routes, with multiple daily BOS/ORD/IAD/EWR, and slightly higher daily frequency on JFK.
However, all this aside, I don't see what the DUB paranoia is all about? Yes, we are leaking pax to them USA wise, but no worse than LHR/AMS/CDG/FRA, so why we single them out is a bit strange.
Just let DUB does what it does, and MAN does what it does. Both airports excell in certain areas, DUB's strong point is USA, but MAN's is primarily going east. We may gift them USA pax but we may get a fair few of their Eastbound pax