Thanks for the advice, guys. No, moneypit, i'm not with Eircom Phonewatch.
We finally got our windows replaced. A installer from the alarm co came out and priced the work at 450 to replace 9 sensors. The original 600 quote was over-the-phone without them seeing the actual work to be done. Now I still think 450 is way over the top - the wiring is mostly intact (no need to run new cabling thru walls or doors) and just needs to extend the wiring and fit the sensors to secure the new window and french-door openings.
I also drew the wiring details before they were disconnected.
I will do-it-myself as a last resort (I have electronic servicing qualifications, so can read wiring diags/schematics and build/wire electronic circuits) but getting the time to do it is a problem. Also, getting details about the alarm system and sourcing the parts is hard to come-by. I think alarm companies like to keep those details to themselves ? I'll check out that link, Brylcreem - thanks.
I'll get 2 more quotes and then decide.