An Analog Telephone Adapter (ATA) is a device that allows you to bridge a PSTN line to a SIP environment like Irish VoIP Call Plans or Hosted PBX solutions.
This means you can route calls that come in over your regular phone line to your IP PBX to allow you to use the advanced features your PBX offers.
You can also route the other way if needed. This is particularly useful if there are particular calls you would like to route over your PSTN line instead of SIP that do not have a route through SIP.
You can also use ATA's in the home to act as a bridge between your own phone line and VoIP. For example, you could set it up so that when you make an outgoing call it uses your Irish VoIP Call Plan and that inbound calls still come in over the regular phone line.
The ATA will do the work behind the scenes to make this transparent to your family's use of your DECT phones at home.