We in America are used to barbed wire whose purpose is to discourage cattle or sheep from wandering across boundaries.  We're not so used to 8-inch barbs intended to discourage war horses from plunging across defense lines.  This is part of several lengths of defense chain in front of the Birr Castle moat.
(However, like the moat itself, this chain was most likely forged in the 1840's as part of the Demesne's famine relief work-for-food program and was never intended to deter warhorses.)