If it's not a Protected Structure, or abutting a PS, then the owner can pretty much demolish at will. Have you checked the RPS of the Dublin City Council Development Plan? That should be your first port of call.
If they intend to replace a single dwelling with a single dwelling, there would be slim grounds for objection- injury to the character of the area or to the setting of a Protected Structure in the area would be one avenue, but it can difficult to prove.
Your best bet would be to engage a planning consultant if you feel very strongly, as they would be able to give you specific direction and advice. In the long run it can be worth the couple of hundred euros fee if it protects the value of your house or your area. Not knowing the particular property in question I can't say much more than that.
You could also make an Observation on the application rather than the standard Objection- a chance to say what kind of development you'd find acceptable.
If you're looking for info on the park, 'Four Roads to Dublin: A History of Rathmines, Ranelagh, and Leeson Street' by Deirdre Kelly might have something, or the local library.
EDIT: Cross post with Graham!