Why, when speaking of the branded product Plasticine, did [do?] Irish teachers insist on using the Irish word marla? Even that word was, according to Terry Dolan’s Dictionary of Hiberno-English [Gill & Macmillan, Dublin 2004; new ed forthcoming], derived from the English marl.
At least in the nineteenth century, marl was a valuable manure or fertiliser and, on Lough Derg, Mr Head of the Derry Estate introduced a system of dredging it from deep water. Read about it here.
Irish waterways history 
Brian,
We spent many happy childhood hols with our country cousins at Rathcabban, 8 – ish miles from Banagher in Birr direction. I actually visited them last week, while on Shannon Rally.
A small river flows through the farm, a Brosna tributary. We used to swim there in what was known as the MARLA HOLE, obviously the same as in your piece. Our feet would sink into the river bed, which had an oozy placticine texture.
Totally seperately, I still play (geriatric) cricket in Leinster Cricket Club, Rathmines. Whilst not now, wickets used to be prepared with “Marl” to give them an even consistency.
Rally was as good as ever.
Jim Murray
Thanks, Jim. I forgot to mention that you can sometimes see a layer of marl under a bog (if you happen to be under a bog …). It was visible on the Rockville Navigation, for instance, although my pics don’t show it clearly. bjg