My cousin was told by our grandmother that her great-grandmother Catherine Lewis was Jewish  but converted to Catholicism when she married an Irishman John Keogh (actually George Keogh).

Little to go on but a reference to Catherine Lewis being the aunt of a Sir George Lewis led me to the family connections listed here.

"George Henry Lewis belonged to a family of Sephardic Jews from the Netherlands who settled in England in the eighteenth century, afterwards changing their name from Loew to Lewis"
(Lewis and Lewis: John Juxon)

An explanation commonly found for the surname "Loew" is that it is the Germanic rendering of "Levi". There are numerous names that are equivalent to or related to Levi: Levi = Aleuy, Elvy, Halevy, Ha-Levi, Lavey, Lebel, Leblin, Levay, Leib, Leopold, Leve, Levene, Levenson, Levi, Levie, Levien, Levin, Levine, Levinsky, Levinsohn, Levison, Levit, Levy, Lewey, Lewi, Lewin, Lewinski/Lewinsky, Lewinson, Lewis, Löb, Löbl/Löbel, Loewe, Loewi, Louissohn, Lovy, Low, Löwy, Lowy, Levinson.