Linguistic experts believe 10,000 words is the minimum. See interesting article at http://www.lang.ltsn.ac.uk/resources/goodpractice.aspx?resourceid=1420
[This article is talking about acquisition of a 2nd language in general - not Irish per se.]

Another discussion of this is at http://www.wordhacker.com/en/article/vocabularysizewordlists.htm, and says that 10,000 word families (not just words) are required.

(Message edited by djwebb2002 on September 27, 2005)