Sounds like Dr MacLysaught is worth reading. That's the most sensible bit of new news on this subjec tthat I've seen for years!

The Norman invasion of ireland was Cambro-Norman rather than Anglo-Norman.

Shhh! lets not mention that, or someone will start claiming Mr Darcy was the descendent of Taliesin and Owain Glyndwr! ;-)

Also of note to the DeBourgh/Darcy connection is that the MacWilliams were a very important branch of the de Burghs.

Which might explain why none of us have found a Fitzwilliam connecion. Perhaps we could impose upon you a little longer, and you could give us more details of this connection?