They're happening ahead of the start of the Climate Summit in Paris tomorrow.
The summit is seen as the last chance for world leaders to reach a legally binding agreement to tackle rising global temperatures.
Here, thousands are expected to turn out at demonstrations in Dublin, Cork, Galway and Belfast.
The biggest march is in Dublin – that's underway from the Custom House at 2 o'clock.
Climate Change Officer with Oxfam Ireland David Healy is hoping large numbers will turn out today.
