Effective 1 November 2011, Malahide Castle is closed to the public. I've been advised they are anticipating reopening in May/June 2012 with a new operator to run the castle tours and other attractions on the property.
If your travel plans include Malahide Castle, check out their web site in advance of making the trip there:
http://www.malahidecastle.com/ . You may also contact their county's administrator George Curran at George.Curran@fingalcoco.ie for current info.