That's excellent information, Lyn & Merlin! Good suggestion re: the Ann Meehan in 1857 also possibly being Owen's granddaughter.
Back at the drawing board, could these be some leads to follow up?
1. Do you know where Owen Meehan was buried or if he left a will (maybe in a probate file?) ? Maybe you could check to see whether there's a Honora buried nearby or mentioned in the will?
2. Another family member not previously mentioned here also was brought out by Owen Meehan on the Simond in 1855: Rose Mul(l)holland age 34 (parents Hugh & Mary), a sister of Owen Meehan & a widow. (see
http://trove.nla.gov.au/ndp/del/article/60179326?searchTerm=meehan and
http://trove.nla.gov.au/ndp/del/article/12968080?searchTerm=meehan). Maybe her whereabouts could add a lead re: where Honora ended up?
3. Possibly a huge long shot -- but I noticed a "Nora Mehan" in Connecticut, USA in 1870. (ref:
https://www.familysearch.org/search/image/show#uri=https%3A%2F%2Fapi.familysearch.org%2Frecords%2Fpal%3A%2FMM9.1.i%2Fdgs%3A004259463.004259463_00009 (4th row))
Sounds a long way away, but she's the right age (40), from Ireland, no sign of any family, and also can't read or write (which is the same as your Honora declared in 1855 when she went to Australia). Maybe worth further investigation?
Update re: Rose MulhollandRose appears to have been married to Philip Mulholland, who seems to have died before she arrived in Australia per the Simond in 1855. Two of their sons appear to have also come out to Australia, in May 1859 on the "Palmyra" - Hugh b.c.1835 (d. 1901 in Ryde NSW) and Philip b.c.1847 (d. 1932 in Hurstville NSW). This information may help you locate Rose? [Also found that, like Rose, Patrick McNally similarly stated in 1855 age 40 that his parents were Hugh & Mary, both dec'd (though he was listed as Owen's step-brother, while Rose age 34 was listed as a sister of Owen).]