Renting apt in Spain: upfront bank draft or 20% deposit & bal on arrival?

Yes, it is normal practice to ask for deposit but you could send a personal cheque which would be more secure. however with electronic transfer you could transfer to his Irish bank(you will have details). With holiday rentals you have automatic insurance against fraud.
If he is advertising with them he is authentic as it doesn't come cheap!
 
We used to advertise our holiday home on some UK websites. We stopped advertising there because (believe it or not) we were too successful. I never intended to make a major profit, I just wanted to cover some expenses e.g community fees, water charges, electricity charges, refuse amounts, public lighting etc (normal taxes foreign holiday home owners incur).

There are scam merchants out there. It amazes me how gullible some people are e.g the report on The Times (above). But, you can take some precautions deal with genuine people, preferably Irish (not being racist here), know where the holiday home owner lives, works, deal with owner only, not representatives.

Be aware that bargain might be too good to be true. Ask questions from the owner, location, walking distance to beach, supermarket etc, accessibility etc.