I lived for 25 years around the southern 'village' area of Portmarnock, the end closest to the current 28 approach, about 1.5km north of the current 28 outer marker. The noise levels are not an issue in my opinion. When it is noisy is when 10 is in use and the aircraft are taking off over Portmarnock. Even so, noise at the north end of Portmarnock (the most built up area and 2-3km further north) is considerably less and of very little concern at all.
It would seem to me that the sensible thing to do under normal conditions is make approaches on the current runway, which will be 28L/10R and take offs from the new northern runway, which will take the departing traffic out further to the north and thus further away from urban areas. More traffic movements perhaps but no increase in noise over Portmarnock.
With an easterly wind, using the current runway for take off (which would then be 10R) will bring the departing aircraft between the southern (less built up) side of Portmarnock and Baldoyle. Approaching aircraft to 10L will again be further to the north than the current approach path thus lessening noise for residents west and southwest of the airfield.
End result, more arrivals or departures for Portmarnock residents but noise from any particular aircraft will be the same levels they currently are. In terms of all the residents in the vicinity of the airfield there would be a reduction in the total noise levels.
Glad I don't live there anymore. Many of those local residents groups made me
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