Hi Florence
I'm not sure if you've tried the Ellis Island site.
There are a couple of Johanna O'Mahoneys but the dates don't match yours; one is from Cork though.
However there is a Johanna O'Mahony on ancestry.com:
http://interactive.ancestry.com.au/7488/NYT715_1651-1198/4013869172?
She arrived on the Baltic in 1911 aged 1; she was a detained alien coming to her sister Mary Hudson.
This is the only one I could find where the age matches; none in 1949 I'm afraid.
Col