He's just popping up to say hello, no need to be alarmed.
A trillion years ago when I was a wee tot I had a mug with a little mouse in it who would pop his head out to say hello when I was about half done with my cocoa. I'm glad to see that there is a classy adult version now that I'm an ancient lady.
Have you started stuffing your face with corned beef and cabbage yet? Are you 4 Guinnesses deep? Have you been pinched or kissed? Go crazy, dye the river green, gorge on Irish cheddar and remember to drink lots of water and take some ibuprofen before bed.