oops, sorry for that really long post there..... i got a bit carried away.
but i just remembered in relaton to the Déisi, i had read somewhere that they might not have been a lower class in Leinster as i mentioned above, (but there are some people who think they might have been) and that the name might have been given to them after being subjugated by the incoming "Gaelic" race in the later centuries BC - ie they were a ruling class, but lost control etc...
also, aitheachtuahta is apparently a common name, and Deissi is not as common, if not common at all
