Moytura, or Magh Tuiread, apparently means 'Plain of Pillars', according to this webpage. They were apparently thrown by the Tuatha De Danann when they were waiting for the their battle to start with the Fomorians. One of the stones spotted by Ryaner has its own name - The Eglone Stone, and has at least two associated folklore stories.
There are many huge megaliths like this around Moytirra east and over on the other side of Lough Arrow at Carrowkeel. They are very probably glacial erratics but this one seemed so like it had been PUT there that I've posted it here. Should anybody have the slightest difficulty with this I'll delete it. I would like the chance to go back there and investigate further, hopefully when the cows aren't around.