Hi Cocksie,
You are right on the money! As you can see, it's a while since I last posted here. I am currently working more on my mother's paternal side and this line is my mum's maternal side. However I have done some more research since the last post, and I had found indeed IGI records that suggest that William and Denis Kelleher are brothers, and sons of Murtagh Kelleher and Johannah Keiler/Keiley, depending upon which record you look at (don't you just love the spelling variations) .
I have Murtagh born abt 1822 in Cork, Ireland and Johannah abt 1826 in Cork Ireland, with possible death of Johannah in Poss 1885, Sydney. Marriage abt 1847 in Cork Ireland.
The problem is with the dates of birth for William and Denis. There seems to be a big disparity with conflicting records in IGI. I have William's DOB anywhere between 1836 and 1848 and Denis between 1838 and 1841. Given that the parents married in 1847 makes matters even more confusing, though I have plenty of birth records that come before marriages. Worse still, looking at the date of marriage of William in 1863, he would have difficulty being born as late as 1848 as this would make him only 15 for marriage
Given the prevalence of the names of Kelleher, Buckley and so on in Cork, it's even hard to know for sure if the IGI records are for the people we are discussing here or others.
There is a death record for William Kelleher 1911, Bowral. Parents as Mortimer & Johanna which I am sure would be him, and one for Denis 11 July 1889, Kiama with father Murtie. I think his legal name was Murtagh and he probably went by Murtie as a nickname. Both William and Denis were twice married but I won't go into further detail here as I am just getting the bare bones off my tree, as I haven't looked into the detail of the records for a while. It's interesting you have found the sister, Mary. That might aid in connecting the family further.
However, my original question still remains: I wonder whether these Kellehers are related to Mary Kelleher who married John Buckley, the nephew of William and Denis?