If you have a Garmin Edge device, adding a course from RideWithGPS.com is easy.
It is not needed, but recommended to create a RideWithGPS account. A basic account is free. Just visit https://ridewithgps.com/signup and do the "paperwork".
You may need to rent a bicycle when you're in Dublin.
Here are some alternatives where to rent a bicycle.
Using your bicycle on the roads makes you a part of the traffic picture. You have to follow the traffic rules and regulations.
Your should be very careful if you're not familiar in left-sided traffic. All your natural behavior of reaction will not be of value if you react to "left" when you should lean "right".