Hi Sue. We filled in a form with UPC and about 4 weeks later the changeover was complete.
Note that UPC actually gave us a new number until we switched-we had 2 numbers for this period, so you are not without a phone at any stage. Their physical line is totally separate to Eircom's-it is only the number that needs to be switched over, so it's pure admin work.
Having said that, we were actually switching from UTV at the time and not Eircom.
When our number was switched to UPC, we noticed the following
1. We were no longer ex-directory (I asked to be put back as ex-directory, but it hasn't happened, it doesn't really matter as we aren't in the phone book anyway)
2. Our phone wasn't working one evening. I logged a fault. They came back and said something like 'oh we were doing some work and we had to reprovision your line etc.'. Anyway, we could make calls out, but not receive calls. They had given us the completely wrong number (not our original UTV/Eircom number, not our temp UPC number, they had taken this from God knows where....) A few calls later and it was sorted out. I am still none the wiser as to what they were up to....
3. Other than some techincal issues with the cable into the house which did take a while to sort out, it's been pretty good. Your phone will be connected to your modem, which is plugged into a cable point, which is something you may have to consider....
I wouldn't worry too much about Eircom and their €120-as long as you have 6 months of phone from UPC, then youy will have saved the equivalent in line rental.
Go for it-if you have any other questions, please do ask!