As rare as leptospirosis is it's good to be aware of it if you're paddling on the Liffey.
Simply put;
- It presents flu like symptoms 1-2 weeks after getting into your system.
 - If it is dealt with quickly it's fairly harmless.
 - Try and cover any cuts you have before getting on the water
 - Have a shower when you get home Something you'd want to do anyway after kayaking.
 - It is extremely rare.
 
If you do have flu like symptoms after a 1-2 of weeks after kayaking on the Liffey tell your doctor, he'll give you a blood test.
The HSPC has a little fact sheet on it 
here.