The details aren't that well explained, but...
The official page on the card itself is here:
https://www.leapcard.ie/en/PageSetti...53h27CkUIxE%3d
(memorable URL, eh?)
It mentions that the card is valid on DART and Commuter Rail within the 'Short Hop Zone', which is shown on this page:
http://www.irishrail.ie/fares-and-tickets/dart
and does include Howth.