Actually, the topic has been discussed several times.
Has it? The Welsh being replaced by Irish? I dont remember any such discussion.
Since the Irish are the only Gaels around at the time, that means you need an Irish invasion. To meet the bonus challenge, having the southern Irish cross to Britain instead of the Northern ones (maybe DalRiada(sp?) is more sucessful IN Ireland?)
Still, getting these Irish to actually conquer and culturally replace the Welsh would be tough. Very tough.
The Picts were more primitive, not christianized, etc, which made the Gaelic cultural conquest a lot easier. Wales is on an equal footing to Ireland, and the assimilation is more likely to go the other way.