Unfortunately, at the time Mortimer Brownrigg Som(m)ers travelled - the shipping manifests wouldn't show more that what you have - the type of detailed manifests showing home address, destination address, relatives/friend from whence came, relatives/friend going to, precise birthplaces, etc., came decades later.
I've had a look for him US Census 1850+ - no sign, and I searched under a variety of ways (Mort*) (Mor*), (Som*), (Brownr*) & so on.
I also looked to see if there was an Eliza Som*ers arriving in NY after him (often when immigrating a man made his way over first, wife children later).
I see from other topics/posts on web that : your Robert Frazer SOMMERS, Draper of Aston's Quay Dublin in 1880 when he married Phebe Letitia JACKSON in Roscrea, Tipperary - gave his father as "Mortimer B SOMMERS, Hotel Keeper".
And when Robert died in 1913 in Australia his youngest son informed Roberts parents as:
Brownrigg Sommers and Eliza Jackson - as you say, likely a bit misinformed or failed/partial memory at play there.
Will keep looking - sorry not to be more help!
Cheers
AMBLY