Things are still a little quiet as the schools are only just back but probably from next week you'd have to be in Malahide for about 6.50 and Portmarnock for about 7.15 to be sure of a spot. We live in Malahide and have a child in a creche in Portmarnock and last year these were the times. My husband would leave the house at 6.40 to get a spot for the train at 6.55 (about a 3 minute drive) and I used to get a spot after dropping the baby to the creche at 7.30 but that's gone now. There is a lot of on street parking (not spaces, just a lot of people do it) on the road up to Portmarnock station but just be aware we were hit with a parking fine there before, the muddy section just outside the station is not legal parking, and cars that park on the path opposite had a day last year where their tyres were slashed. Other than those 2, the rest of the northside stations on teh way in don't have car parks, except for Clontarf but the drive that far wouldn't be great. Sutton and Howth both have carparks but I don't know much about them. I've heard the Sutton one is better since they've opened an overflow, but the problem approaching it from teh Portmarnock side is that you can easily get caught with the crossing gates down and miss a train.
I now get the 32x from Portmarnock at about 7.35 in the morning and usually get off at Trinity at around 8.10 if that's any help.