The second bridge in Donard Park is closed for rebuilding. It looked like they had made provision to cross the river just above it, but the rivers are crazy at the moment so no way across.
If you are heading up you are best to stick to the right of the river the whole way up to avoid crossing. There is a trail through the forest just past the first bridge if you do not want to wind up the fire road.
Most of the paths are rivers as well up there. Some amazing waterfalls too, just dress appropriately: soaking and cold.