Since I have lived in the Chicagoland area my entire life and have Irish ancestors that came right through Ellis Island on their way here, I am legally obliged to mention St. Patrick's Day in this post. After all, on March 17th, Chicago becomes Little Ireland for a day (or a three-day weekend this time), everyone wears something green even if their Irish roots are barely evident, and not only does everyone serve green beer, but they even dye the Chicago River green. This is a real event out here, and the city won't let you forget about St. Patrick and all the great things he did like driving the snakes out of Ireland.
In a post I did a while back, "The Tigers of Africa," I discussed how sometimes facts can get in the way of a good story, and we need to hold ourselves accountable to sticking to the truths on the ground - like how there are no tigers in Africa, for example. However, in recognition of St. Patrick's Day, I thought it might be worth mentioning that sometimes it's not about the factual accuracy of any particular event, but rather what we choose to believe about it, or what people believed at the time.
I'm not going to pick on old St. Pat - he did plenty of things worth note. The only downside is that over 1,500 years, very poor documentation left a lot of wiggle room about what really went down and how much substance underlies the story. For example: most people grudgingly accept that St. Patrick didn't actually drive the snakes out of Ireland - glaciers did. Now, whether the snake story is more a metaphor about driving away pagan beliefs, a supporting anecdote relating to other fabulous stories about his adventures in Ireland, or just creating a fable about why there are no snakes in Ireland is inconsequential. The bottom line is that people still discuss it, and when we write about it, we can throw that little bit in as common folklore. It doesn't have to be supported, strengthened, or even true - it just has to be believed. Now, if you write about this actual chasing of the snakes happening, you might be held to a higher standard. However, most people hear the factoid, connect it to St. Patrick, and move on. And that's okay.
Now, if you've ever been to Ireland, outside the cities it's very green and lush. Granted, most countries in the northern temperate zone are, but Ireland is very much so. So much so that people think it's the national color. Well, not so much, though all the people wearing green might lead you to believe so. That whole bit goes back to Gaelic traditions, the beautiful countryside, and the aforementioned shamrock. But the only important part you need to write about is that people associate one with the other - then move on. The fact that Ireland's official colors are green and blue and two-thirds of the Irish flag isn't green isn't important. What people believe is the part that makes for convincing writing.
Facts are something you have to contend with when you write, but keep in mind that writing about people and their beliefs can often steer wide of the truth, and that's okay. Maybe your character happens to know a lot about St. Patrick - that's fine, use it to enlighten your readers. If your character only believes the old south-side stories and thinks St. Pat was the first leprechaun, well, go with that for fun. Just remember how often facts can interfere with a good story, and as a writer, it's up to you to make sure the story wins out in the end.