Although the upper end of the rental market in Monkstown for a 2 bed is 2300+ per month. Assuming 11 months occupancy at the top end of the market and ignoring stamp duty you would get a yield at 520k pretty close to 5%...
The average rent on a 2 bedroom apartment in Monkstown is €1587.50 based on 6 apartments on daft today.
So purchase price of €520,000 and rent of €1587.50 = gross yield of 3.0%. Miserable return!