The Office of the Director of Consumer Affairs has examined this issue in a recent publication entitled '[broken link removed]'?
Their advice reads as follows:
"If you receive an invitation to participate in any type of lottery, sweepstake or prize draw or news that you have won a free holiday and are required to make a payment or purchase to claim your prize you should be very careful.
Often these companies give only a P.O. box number and no geographical address, therefore you have no way of contacting them if they will not respond to your communication by post.
For prize draws you may be required to call a premium rate number in order to claim your prize. This could result in a long phone call and possibly the need to make several other calls before your prize can be won. All Irish premium rate numbers begin with 15."
There is more information on what to do on the ODCA site - I suggest you follow the link above. If you have any further questions you should contact the ODCA directly - ([broken link removed]).
Hope this has been of assistance.
CMCR