Removing the roof reduced structural integrity of the sides, so there was also bracing installed from the roof level of one side to the floor level of the other - as shown by this interior shot from the Robin N Clements collection on the IRRS Flickr: https://irishrailwaymodeller.com/topic/3251-turf-carriages/#comment-270174 (IRRS membership required to view).
There's a few more shots of the turf carriages in that collection if you're interested. And if you want to mix your turf trains up a little, there's also an interesting shot of a more conventional turf wagon (to my bogman's eyes anyhow): a GSR flat, complete with with creels, fully loaded in Inchicore: https://www.flickr.com/photos/irishrailwayarchive/53507832482/in/album-72157661623942928/