As a "toast" rather than a "Grace", I like this one:

"Here's to the Harp of Ireland, and may she never lack a string as long as there's a gut in the Peeler".

Just in case anyone doesn't already know this, "Peeler" is an old term for a policeman (from the politician, Robert Peel; whence also the fairly polite term "Bobbies").

For a "Grace", this one, "The Selkirk Grace" (often attributed to Robert Burns) is humorous and comprehensive:

"Some hae meat, and canna eat,
And some hae nane that want it;
But we hae meat, and we can eat,
And sae, the Lord be thankit"

And one he really did make is witty:

"Oh Lord, since we hae feasted thus,
Which we so little merit,
May Jock nou tak awa the Flesh,
And Meg bring in the Spirit"