That idea doesn't make much sense to me.

I don't know of any Celtic festivals that take place routinely on the 17th of March.

We don't know that St Patrick's Day was celebrated in any capacity until the 9th century, by when the island was entirely Christian and had been so for some time. If it was previously a Celtic religious festival, why would there be any need to change it after they'd been Christian for so long? And it wasn't even made a Feast Day until the early 1600s.

Patrick wasn't immensely celebrated just after his death, there would be no reason for anyone to immediately declare a holiday around him.

My sources are St. Patrick's World, The Christian Culture of Ireland's Apostolic Age written by Liam de Paor, and the Catholic Encyclopedia's entry on Luke Wadding.