Definitely NOT the Brian Boru. That's an evening only jacket-- I should know, it is one of several Irish designs that I did for Gaelic Themes.

The Crail with waistcoat, white shirt and tie would be perfectly acceptable (it is, after all the Highland equivalent of a suit), although if kilted I would suggest that you buy, beg, borrow, or rent a black Argyle jacket and 5 button waist coat as this would be the same as ordinary morning dress. Wing collars are no longer worn during the day, so stick with a plain white shirt with a fold over collar.

But above all, polish, Polish, POLISH your shoes until they look as though they have been carved from obsidian.

And, of course, enjoy the Garden Party.