It sounds like you have such a special connection with your best friend and I'm so glad you got a glimpse of Irish culture thanks to him.
There are so many gifts ideas for St Patrick's day, or to celebrate friendship in general.
I can think of a few gifts made in Ireland that would be suitable, such as a Claddagh ring
(which are given as tokens of enduring relationships -- platonic or romantic), a traditional Aran woolen sweater
, or maybe a bottle of Irish whiskey
or gin if that's what he likes. A traditional instrument like a bodhran or a tin whistle
could also be great if your friend likes music.
The shops hyperlinked can usually ship to the United States, and drinks made in Ireland would be easy to find in your area.
I hope this will help you narrow down the perfect gift and wish you a great day ahead!