So my good friend, Julie Cabitto, recently found out about her family in Ireland. We share information a lot. She probably knows as much about my family as I do, and vice versa. Her Irish ancestors were weavers, and the area they are from mostly produces linen. Ever heard of Irish Linen? That's her family! One website I was reading gave the history of Irish Linen and mentioned the Huguenots' influence the in the production.
The Huguenots, especially one Louis Crommelin, started arrive in Ireland just before 1700 and hung around until 1828 when the Linen guild changed policies to help industrialize the industry. At their peak, they numbered around 500 families...
What if Marianne Moore de Barcy is of French descent but was born and raised in the Huguenot colony in Ireland? Hasn't there always been religious unrest in Ireland...ever since Henry XIII & his daughters, Mary & Elizabeth, kept the countries' religions flip-flopping? Marianne was born 1835 according to the best estimate we can get. That isn't too far after the 1828 changes. Moving across a channel from Ireland to Liverpool seems a whole lot more likely than from France all the way around England to Liverpool. It fits more with the religious persecution, too.
Definitely needs more research...but maybe a theory to keep in mind?