Luas or 90 bus to city centre (O'Connell Street) then the 3, 11/11a/11b, 13/13a, 16/16a, 33, 41/41b/41c to the stop outside Drumcondra station (or optionally the train from Connolly to Drumcondra) and walk from there
its not the easiest place to get to from Heuston.. i'd say get the bus or luas to O'Connell St.. then a bus to Drumcondra (eg: 3 or 16) and walk down Clonliffe road.. its not a far walk from the city center - but depends how well you know the area.. theres also a map on that website
Bodies was good.. I was with my 11 year old and she asked lots of interesting questions!.. might be a bit creepy for younger people thou!
Zoo (around €25 for adult + child) & Phoenix park might be nice if the weathers good - take 10 bus from O'Connell St. or one of those Dublin Tour buses.. or wax museum/cinema if it rains?
If you're getting the train and can be flexible time-wise you can get 50% reductions on the usual fare — but you have to book online. Alternatively, depending on where you're travelling from, you might be able to get their special [broken link removed] (€5 child return)? Their website also lists a number of other [broken link removed].
If it's raining, there's a big [broken link removed] in Collins Barracks (200m from Heuston) while the main Natural History Museum in Merrion Square is closed for refurbishing.