According to Wikipedia, there have been no snakes in Ireland since well before St. Patrick's era, however, it may be a reference to snake symbolism in Druid culture at that time.
I'm trying McSorley's Irish Pale Ale this year. I was going to get some Guinness, but I went to one Kroger's and they didn't have any 12-packs of anything Irish. I went to another, and all they had was Killian's, so I got some of that. Fuckin' Kroger's.