Yeah, I know, this is a Howth page, but I figured I'd share my personal experience, which was to experience both Howth and Malahide on the same day trip.
You can get to Malahide from Howth on the DART by changing at the Howth Junction Station, but fortunately, Peter was there to drive me. He took me along the coast through a hard downpour, which made for an exciting ride.
Once in Malahide, we headed straight for Malahide Castle where there is also the Fry Model Railway Museum. This place goes through the entire history of rail transportation in Ireland. It's a huge model train that you walk around and listen to an automated guide as you make your way from station to station. I thought it was pretty interesting, but if you're pressed for time, the castle is a better choice.
It was 10 euro as a combined ticket with the castle. You can also combine the ticket at the Dublin Writer's Museum and the Yeat's House.
As you probably have heard, the Fry Model Railway is no longer operating at Malahide Castle. To avoid disappointing visitors, it would be helpful if references to the Model Railway be removed from your website.