Briar is "Queen Mary's Rose" - on a short list of plants dedicated to the Virgin Mary. And the rose is a fairly universal symbol of her as well.
Much ado about St. Mary's Church but it all makes perfect sense to me. Lord Levett has a special devotion to Mary. Not uncommon for sailors.
Not to mention this business of plants laughing as they tie a true lovers knot. That' s about as Irish as it gets :-) I don't believe I'm reading too much in. I am simply analyzing content. It's there...