My G Grand parents were Michael Joseph and Elizabeth Pearson.
A couple of finer details.... The ship that Hugh and Isabella came out from Ireland on was the Navarino. Not the Navarina. I am confused about Hugh's date of birth. His obituary said he was the ripe old age of 91, died in Aug 1892, which of course makes it 1801. But the passenger records and emarkation says he was 26 yrs of age and the year was 1837. Which make him the same age as Isabella, born 1812. A couple of things.... would Isabella really marry someone 10 years her senior in those days, when the life expectancy was so much less? Then they had 10 children. That in itself would be unusual in those times, commencing a family of 10 children if you were 36 years of age. Puzzling.

And Hugh and Isabella's last child, Hubert, was born in 1851.
If Hugh was born in 1801, then he was 50 and still having a child.
I would have thought that to be very unusual. But of course possible! Were the passenger records correct or his obituary?
It is feasible that he lied about his age to the emigration authority, to make himself look younger as a labourer, as he was applying for free passage. It would be good to find some other substantiation.

I haven't been able to find a birth certificate for Hugh in Ireland or a marriage cert for Hugh and Isabella.

After receiving your message I went down to where I believe Hugh & Isabella were buried. (I live close to the Morphett Vale Catholic Cemetery.) Sadly there is no longer a headstone for either of them however your g.grandparents grave is visible although in a sorry state of disrepair. Hubert Kenihan's grave is also there.
I think it quite understandable (& probably common) that Isabella Kelly married a man 10 years her senior in Ireland before leaving for Australia. (Perhaps Hugh was married before? Lots of women died in childbirth as we know.) The fact they also produced 10 children was very common in those days. Hugh seems to have been a fit & active man - producing a son at 50 & living another 41 years -says it all I feel.
As for his birth date - I think it most likely that his obituary would be accurate rather than the passenger list. I would have thought with the variety of accents etc. he may not have been understood when he applied for the voyage. Or perhaps as you said - he may have thought it best to lower his age. For some reason I believed that he paid for his passage out - but could be wrong.
My sister also looked for certificates in Ireland but found none. Sadly this is another common occurance & even at the cemetery here - records are missing. I know my g.grandmother Catherine (nee Kenihan) is buried at the same cemetery but with no records or headstones still standing - it's frustrating to say the least.
P.S. If you are interested I taken photos of the graves. Let me know if you would like me to send them to you.

Do you perhaps know anything about more Hugh Kenihan, who is possible linked to my gr. gr. gr. grandfather.

I'm looking for someone around the time of 1800, to connect to a Joseph Kenihan (born around 1835-1845), he had a son named John Henry Kenehan (when they moved to Canada they wanted to make their name more Canadian). John Henry had a kid named Jack (John) Kenehan, is my grandfather. My father is Jack Kenehan, who was named that because my grandfather didn't want to carry on the "John" name.

From looking at some records I found that Hugh is the son of Michael Kenihan. And Hugh is Joseph's brother, but it's not the same Joseph.

http://www.jim-howe.com/genealogy/kenihan.htm#I20

I did not know Hugh's father was Michael or about his brother. Thank you for the information & sorry I can't help you.
Good luck with your tracing.