VS St.Patrick's day

Now it doesn't deserve to be compared with Christmas day and even with Halloween day, but St.Patrck's day actually and steadily seems to become popular in Japan.
March 17th is the day. Irish people celebrate St.Patrick, who was the first person brought Christianity into Ireland.
I heard that everybody have to drink Irish beer Guiness all through the day on this weekend.

So, I went to Irish pub in Machida last night.
I went to the pub soon after it was open, because I wanted to drink in a happy hour. But, the day was special. All time was a happy hour.
When I went to the pub, there was only one customer drinking beer. But most seats had already been reserved.
And soon the pub became full.
I started with,of course, Guiness. The pub was decorated by green, Irish color, and Celt music sounded there. I don't know why, but a wolf man movie was on the TV screen in the pub. I thought it was fitted to the Irish festival mood.

I could enjoy the day fully, but the only one thing I don't know remains. How does St.Patrick's religious matter relate to the beer drinking? I read the explanation for the day in the pub, but I couldn't get enough information.
In Japan, the new year day has a religious meaning the most. Japanese drink sake in a temple or a shrine. The sake has a religious meaning then. People think it makes us clean.
I don't know Guiness beer has such a meaning, but the relationship of Religion and Alcohol is understandable, and even in the case that they have no relationship, I still continue to drink. Happy St.Patrick's day.