If you know any second level teachers, they should be able to point you in the right direction of someone. happens a lot in my school. approach a 5th/6th year reliable good student and ask them if they would be interested.
I got someone from a local creche. I just called in and asked the manager if there was anyone who did babysitting. You could always ring and enquire, and ask can you drop an ad with your number on it for them to put up (like if the manager is too busy to try and organise it for you).
At least then you know who the person is, and that they are experienced with babies.
If you go to a mother and toddler group, you could always ask around who someone else uses.
As a first time parent I'm in the unfamiliar territory of looking for a baby sitter. Where do I start?? I'm based in south Dublin and am wondering where do people get someone they trust to leave their baby with?