It is possible that Patrick and John were brothers, or related at the very least. I don't think the name Hickman is very common...well in Ireland in general, but there maybe a cluster around the Sixmilebridge area. I have heard there maybe some more around Limerick, but you know about these family stories!
Anyway it's hard to get information on ireland from here in New Zealand. Since my Michael died young and all his children came to New Zealand, it's hard to get family history. Anyway one day we may find the link, or find someone who knows more about the Hickman's from this area!!!