We have Eircom Phonewatch with wireless sensors and have replaced the batteries ourselves. We took one out and brought to our local electrical supplier and he had them in stock.
You could also try ordering online or check in the electrical section of boards.ie where they talk about all sorts of alarms.
Are you talking about the main backup battery in the main alarm box? These are usually Sealed Lead Acid (SLA) batteries... typically 12V 7AH. Have a look and it will be written on the battery.
You can find them here: www.radionics.ie and you can drop in to collect.