If you're moving around within Europe the voltage / frequency are the same 230V 50Hz. At worst, you may need to change the plug.
Analogue television is rapidly becoming irrelevant but, many televisions, as has been pointed out above, will work in multiple countries. You usually have to select the country you want to set up for in the installation menu.
However, some models, particularly those sold in the UK, may have quite limited tuners that only tune into a narrow range of UHF channels. This can mean that they won't work or may not receive a full range of channels even in the Republic of Ireland.
Some televisions also do not contain SECAM decoders, so will only see French TV in black and white!
Digital TV in Europe is all broadcast using DVB-T, but there are some quirks in that too.
E.g. some countries use MPEG2 for standard def and MPEG4 for HD only (E.g. UK) and others use MPEG4 for everything (E.g. Ireland and Norway).
So, you may need a TV that is capable of decoding MPEG4.
However, many people don't watch TV using the set's built-in analogue or digital tuner.
If you're using a set-top-box i.e. cable, satellite or digital terrestrial it should work just fine using either a SCART or HDMI cable.
Televisions outside the EU (and European influenced areas) may be drastically different e.g. the United States, Canada and Japan etc.