The Tara Brooch - I have not been able to find a really good picture of the broken end of this chain. This book has the best picture I have seen of it. It appears that the chain was pulled to break it, and the diameter decreases at the broken end. This would happen with a hollow tube, but not with something like loop in loop. The Brooch is housed in the National Museum of Ireland.
Page 90 - S link chain made from flat metal and connected end to end.
What Life Was Like Among Druids and High Kings, Celtic Ireland AD 400 - 1200 by Time Life Books, Alexandria, 1998. ISBN 978-0783554556.