Ha Ha!!

It seems an explanation is in order!!

A 'ha ha' - at least in Northern Irish parlance - is a ditch! The gentry used to look out of their lovely homes and see the rolling countryside all around them, and enjoy the sheep and cattle grazing from afar. What they didn't like was same said sheep and cattle trampling their plants and digging up their lawns - so they dug a 'ha ha' which was a long trench along the boundary of their gardens. From their stately homes you couldn't see the ditch, you just continued to have your lovely uninterupted via over the countryside - but the sheep and cattle on the other side of the ha ha then couldn't get into your garden to wreak havoc with your topiary!!

Hope this helps!! I have linked to Florence Court on Wikipedia - where there is a particularly fine example of a ha ha, which sparked my original comment yesterday!