Hi,
The 102 feeder bus runs from Yellow walls in Malahide to Sutton station. Not sure if it diverts to Portmarnock Train Station. Have seen a bus before in Portmarnock stations but is a long time since I have used it in the morning. You'll need to contact Dublin bus on that one. However, pretty sure the 102 still does not run on Sundays.
If you want to use Portmarnocks station, usually you get off the bus at the stop just after the roundabout at the bottom of Portmarnock and walk up.
The 230 (bus from Portmarknock to Airport) has the terminus at the Portmarnock Roundabout as well so could use that as well. The 32B/32A only go the coast road but will also stop at the roundabout in Portmarnock.