17th-century Dutch maids were probably complicit in the ferrying of love notes for their employers.

At least Gabriel Metsu, and many painters of the Dutch Golden Age, so thought. While literacy was on the rise, it was still a privilege back then, so using maids to pass your letters would make perfect sense, as they probably wouldn't be able to read the contents even if they peeked. It was like surfing the web in incognito mode.  The maid here definitely wants to know more. Her mistress, on the other hand, is focusing on the letter, which she tilts toward the window for better light and more privacy.  Deducing from her intense concentration it looks important. 

How are we so sure that it’s a love letter? Metsu created this painting to accompany Man Writing a Letter, which also lives in the National Gallery of Ireland. Once you see the guy who is writing to her, you’ll know. They look like lovesick teenagers! The content of the letter is illuminated by the painting of the ship sailing on rough waters in the background, which most likely refers to a common simile likening the course of true love to choppy waters. The maid has another letter in her hand, but that letter isn't part of the narrative--it's just where Metsu humorously placed his signature. 

While Metsu is often compared to Vermeer, and they likely knew each other's work, there are differences between the two artists. With Vermeer, we stand behind a curtain and glimpse a scene. His works are cryptic and quiet, often with the barest hints of narrative. Metsu, on the other hand, gives you a lot more clues. He was all about the puzzle. You fit it right, you find the story. Well, the way our brains work, we can fit almost anything into a narrative. Perhaps the lover is going to sea. Perhaps the dog is a symbol of faithfulness. Whatever is going on, the maid probably knows a lot more than we think.