I got connected to castle garden, I didnt check for John Cahill but often emmigrating pairs met as passengers and fell in love etc. and by looking up the ship you can find who else travelled at the same time another relative etc. you will need to look them up yourself as I have taken out the ship name
Margaret Duffy Child, Youngster 14 F 22 Jun 1846 Ireland
Margaret Duffy Unknown 21 F 3 May 1849 Ireland Margaret Duffy Unknown 16 F 18 Jan 1848 Ireland Margaret Duffy Unknown 18 F 22 May 1847 Ireland
Margaret Duffy Unknown 34 F 25 Jan 1848 Ireland
Margaret Duffy Child, Youngster 9 F 5 Jul 1849 Ireland
Margaret Duffy Unknown 11 F 7 Jun 1849 Ireland
Margaret Duffy Child, Youngster 17 F 26 Jun 1835 Ireland
Margaret Duffy Domestic 18 F 15 May 1846 Ireland
Margaret Duffy Farmer 18 F 30 May 1849 Ireland
Margaret Duffy Unknown 23 M 24 May 1847 Ireland
Margaret Duffy None 25 F 22 Jun 1846 Ireland
Margaret Duffy Servant, .. 30 F 28 Jun 1849 Ireland
You should be able to narrow it down with her age at marriage Good luck, Jim