Unfortunately there are scammers willing to take advantage of people in this overstretched rental market – in Dublin particularly. There’s a link to an excellent article from The Journal.ie below, but some basic steps are:
Check the RTB One Stop Shop website register of tenancies. If the property was let previously, it will appear here. BTW – Room Share in an owners property may not appear on this register.
Check the PSRA website. If the person advertising says s/he is a property agent, s/he must be registered with the PSRA.
If the advertiser asks you to pay cash, ask if you can transfer to his/her account instead.