Joe's right: Lent is one thing ~ readily ignored when in conflict with St. Patrick's Day, as is normally the case ~ but the final week of Lent, Holy Week, is something else altogether, something to be taken much more seriously.

Of course, in the US at least, not all celebrants of St. Patrick's Day are especially devout Christians. In the US, most are probably at least nominally Catholic, but I would wager that huge numbers are of the "fallen-away" persuasion.

Incidentally: I've seen and heard claims that more US citizens of Irish descent are Protestant than Catholic, which at first I found very hard to believe.

But I've figured out a likely explanation: very many pre-Revolutionary American settlers, especially among the indentured servants shipped to Georgia and the Carolinas, were "Scotch-Irish" Ulster Protestants. These folks had already been multiplying, assimilating, and intermarrying with Anglo-Saxons and others for several generations before the potato famines propted large numbers of Gaelic (Catholic) Irish to start moving to the States. This more recent immigrant group has always been more attached to their homeland, staying in touch with relatives who stayed behind and in many cases sending money back to Ireland to bring more family members across the ocean, etc. These folks, who identify as "Irish" much more strongly than the decendants of those Ulstermen who had been brought here forcibly a century or more earlier, are the constituency for St. Patrick's Day celebrations.