What hardware do I need to have a wireless light switch in UK/Ireland?
I found a couple with simple search for "3-way wireless switch", 
Leviton 6696-W Wireless 3-Way Decora-Style Switch Kit



and BL-6133-WH Wireless Add-On Switch Set.



Unfortunately, both are listed for 120V. Though, I'm sure there is something similar available in your area (which I'm assuming is 220V).

For the two I found, installation is simple. Swap out the existing switch with the receiver switch, then install the transmitter switch wherever you need it.