I would second Greg T's opinion on the food at County Louth. The dinners I have had there have been fantastic. Service was terrific as well.
Adam,
It really depends on how you like to do things. I tend to like to be close to my next round of golf so I personally tend to drive after golf so as to have a relaxing start to the next day.
If you are flying into Dublin in the A.M and then playing Portmarnock and then The Island I would personally suggest moving on up to County Louth, having dinner and staying there. Like I said, the dinners I have had there have been fantastic. In fact, every time I am there in the afternoon I stay for dinner. Back tracking to Malahide and then making the drive up to County Louth the next day is a bit much for me personally. The 20-25mins it might take to get to Malahide would be about half the time it would take to get up to County Louth. So I would just do the extra 25 mins and save a 50 min drive the next morning.
Malahide is a very cool place to spend time though. Very cool place.
Hope this helps.