there's a festival on saturday (14th) http://www.festivalcelta.com.ar/
but I'd prefer to celebrate on the day itself (17th)
some info here: http://www.sanpatricio2009.com.ar/actividades.htm
should be something on here: http://www.matiaspub.com.ar/
and in the Kilkenny.
That area around Retiro...