Your location in the town won't actually impact your e-fibre speed as the service doesn't come from the exchange.
What eircom are using is known as FTTC (fibre to the cabinet). Basically, there's a green coloured cabinet sitting on the street which contains the equipment your phone line's connecting to for broadband. That's connected back to the exchange with a number of fibre optic cables.
You then connect to the cabinet using a very high speed version of DSL called VDSL2.
The distance between you and the cabinet is what matters as the technology being used over the phone lines is very distance sensitive. If the cabinet's extremely near by, you'll get close to 70mbit/s (and beyond when vectoring gets switched on this year).
If you're further away it drops back to maybe more like 40-50 mbit/s
There's a crowd-sourced map on boards.ie's broadband forum
Check it out and you'll be able to figure out the exact locations of the cabinets in Carrigaline and then decide where you want to rent/buy, if broadband speed is a deciding factor.
The only thing I would warn though is the wiring doesn't always take the most direct route back to the cabinet. So, even if it's near by, you might find the wires don't run the way you'd expect and could be much longer / shorter than you'd think!
It looks like there are two cabinets in Herron's Wood area, so I'd be surprised if you didn't get good speeds.
I'm not sure how accurate that map is either, so just look out for the cabinets (There's a pic of them on the link above) and they're all exactly the same. So, you'd be looking for a smallish green cabinet with some distinctive features.
Bear in mind that the rollout's still ongoing, so even in areas that are marked as 'complete' they are continuing to add new cabinets and also 'vectoring' is being switched on which will see speed increases across the board. This is a noise-cancelling technology that allows them to squeeze more bandwidth out of the copper phone lines from the cabinet to your house. Giving you speeds of more like 100mbit/s or so.