What a great use of old Christmas trees. These are placed along coastal or river regions experiencing erosion and help the native grass (maram or otherwise) to take hold again and keep the bank together. The example in the link is for the River Dee and fisheries board - I have not come across many examples of this applied in coastal regions in Ireland although plenty online for the US. I wonder is their any harmful side introducing that many Christmas trees to a river area - like changing the pH of the soil. Possible that only happens with live Pine.