There are two issues here.
One is the rate you are offered and the other is any minimum dealing costs which can hurt for small amounts.
I'm not aware of any websites that provide a comparison of FX costs for ATM machines.
However I am delayed at Gatwick airport and sat by the money corp desk.
Their spread for euros is currently just under 27% so you should certainly look to change outside the airport.
For small amounts have a look at the an post fx MasterCard service.
The rate isn't the best by a long shot but you avoid the fixed charges.