This is a really interesting. There are a number of people and
projects you might be interested in getting in touch with.
Conor McGarrigle has made a map of NAMA owned properties
http://www.stunned.org/2010/10/11/namaland-2/
Richard Cyganiak
Has been working visualising council planning applications
http://lab.linkeddata.deri.ie/2010/planning-apps/
Also someone gave a go at crowdmapping
http://twitter.com/#!/lostexpectation/status/23145612841058306
http://crowdmap.com/
Here is the Open Data Ireland Group
https://groups.google.com/group/open-data-ireland
Some of the projects above are done part time or failed to take off
for other reasons. That is not to say they are not worth revisiting
and seeing if people are interested in taking things further or
developing them in a new direction.
Mapping empty properties is interesting for a number of reasons.
I was at a meeting with some activist groups a number of weeks ago and
a discussion came up about finding and using vacant spaces for
cultural and community groups and small businesses.
It may be interesting to combine this with something like the Culture
Hack Day project to map Irish cultural data
http://www.mindmeister.com/82209607/culture-hack-day-ie
http://culturehackie.posterous.com/
http://dho.ie/drapier/
Apart from mapping there are a number of other interesting projects
looking for developers
http://reformcard.com/
https://groups.google.com/forum/#!forum/reformcard
http://dublinstreams.blogspot.com/
Otherwise you can meet like minded folks through the Irish Hackerspaces
http://hackerspaces.org/wiki/Ireland
Hope this is of some help
Kevin