I'm not 100% sure about NIE standards. For most things they do follow the UK standards, however, you have to realise that NI has never had any direct AC interconnector to the rest of the UK. (The Moyle interconnector is very recent and is DC)
Thus, it has never really been a big issue for NIE to use UK transmission voltages.
However, until very recently, there were also no North-South interconnectors due to the Northern Ireland terrorism problem. The main ESB-NIE interconnectors were targeted by various groups in the 1970s.
So, right through the 1970s/80s and most of the 90s there were no ESB-NIE connections.
ESB's transmission voltages in the republic are:
400kV, 220kV, 110kV and 38kV (sometimes considered distribution)
As far as I'm aware the bulk of the NIE system is 110kV also.
I've also heard that Northern Ireland standardised distribution voltage at 230/400V years ago, perhaps before the UK 240V was picked.
I haven't really had all that much experience of NIE's network components e.g. lines/transformers, but to be honest they don't really look very different to ESB equipment in the Republic.
You can sometimes see where the border is however as all lines (telephone and power) suddenly stop on both sides.
The ESB's pole mounted smaller transformers tended to always (and still are) can-shaped. Similar to what you'd see in the United States in shape/size. You do seem to see more "blocky" looking transformers on NIE poles.
The distribution voltages in the Republic: 10kV, 20kV and 38kV may also be different to the North.
If UK voltages are used, it's more likely that UK style xformers are mounted on poles.
Lyle Dunne or someone in Northern Ireland might be able to enlighten you more than I can!
Also, both companies use major international suppliers like ABB, Siemens etc for transformers so, it's quite likely that regardless of voltage the actual units will look extremely similar, particularly in more modern installations.
ESB's been upgrading rural network in a pretty huge programme over the last few years. So, there are very few old xformers left. They're almost all newish ABB units, still can shaped though.
The telephone installations north and south of the border are DEFINITELY different. You can clearly see BT installations in NI and Eircom installations in the Republic. The wires are different looking, the poles look a bit different and the pole mounted equipment is totally different.
Also, the usual give away:
The telephone boxes are different
The best way you'll know you've crossed:
Northern Ireland : UK signs / road markings (all white lines):
Speeds/distances in miles
Triangular (euro style) warning signs.
Republic of Ireland:
Yellow line marks hard shoulder / edge of road
White line divides lanes
US/Canada/Aus style yellow chevron/diamond shape warning signs.
All speeds and distances in KM.
Direction signs marked bilingually.
I'll have to look at the poles/pylons though more closely next time i cross
[This message has been edited by djk (edited 01-08-2006).]