Just to correct you on a previous point re: Ireland's phone sockets.
P&T (now sold off and called Eircom) introduced RJ11 as the standard jack in Ireland in the late 70s/early 80s during their first phase of digitalisation & upgrading of the network.
It's wired like the US with the signal being carried on the centre two cables. If you plug a US phone/modem into an Irish socket it will work.
The other contacts may not necessarily be used for the same purposes as the US though.
this link gives the full official specs: http://www.eircomlab.com/pstn_pub/pstn.pdf
There was a 3rd wire bell configuration in older installations but it hasn't really been used for a long time. Some configurations also exsist that use the outer terminals on the RJ 11 to trip out other sockets during a call. I'm not sure how it works exactly.
Touch tone/modern ringers have been the norm in new installations since the early 80s. Older installations would have used "headphone style" jacks or have been hardwired.
Using UK specified phones in Ireland:
Generally BT branded phones in particular use RJ11 connectors on the back of the phone that carry the signal on the outer two pairs. In order to use them you generally need to use a BT to Irish adaptor with a capacitor in it. (they're small and cheap and readily available)
Other UK equipment can be used simply by plugging in a normal Irish phone cord in place of the BT style one. 3rd wire ringing requires the adaptor with capacitor however.
Caller Display is also different. Eircom uses a long first ring to activate caller display equipment where as BT inverts the line polarity.