Not sure what you mean by "kind of special"
A relay is exactly what you need, use the 24v into the relay to switch a 230V feed to the light.
The 230V into the transformer will almost certainly be a fixed live and not switched in anyway. (though you could use this for the 230V feed to the relay)
This is the sort of thing you need
http://www.maplin.co....10a-power-relay-37514
(the bottom one, Order Code: N12AW)
Although the fact you are asking the question indicates you probably shouldn't attempt to do this yourself.