Since I play this song, I decided to check some sources. Ha!

I found this page, which explains about Irish language dictionaries online.

http://www.teanglann.ie/en

I searched for SI in the proper dictionary, and they say it is not found. Another site, a 'terminology dictionary', says that SI can be Spark Ignition, Silicon, or Social Intermediary.

As far as the song goes, I know less than ever.