It depends on the ISP and how they break up their network, but most ISPs wouldn't provide that level of detail in their allocations.
So, for example, there's no way to know if an Eircom IP is from a machine in Dublin or Carlow ..
What I am trying to do if figgure out how to show certain content on a site based on County or town
So for example to show the right county flag
So if some one from clare logs in show the clare flag
If dublin show dublin flag
If I can do this with counties or better towns in Ireland
I can do this with countries but not counties
Eircom were pretty good at making sure that if you were in Castletroy, Co. Limerick, your IP (static not dynamic) would reflect that but increasingly, the town/county accuracy isn't reliable, mostly showing as Dublin for example