The first thing you need to establish is whether or not you can be served by upc (new name for chorus, ntl). I switched my eircom calls and bb last year and bundled with ntl, and I find it good value. LOVE my dvr (apart from the times it freezes and does strange things). Now pay about 69 euro a month all in, whereas it was over 120.
The physical line into your house is terminated and if you want to keep your number, it'll take about two weeks, but you can make and receive calls in the meantime. ntl also provided two new handsets which were a saving as I would have had to buy these.
If you can't get ntl, then use callcosts.ie to have a look at the best package for you given your call profile.
Personally I don't mind having all from one supplier.