Please help if you can!

My great grandmother: Mary Ann Thomas HUDSON (parents Thomas and Frances (nee Sullivan) was born 25/7/1872 in Douglas.

She was baptised on7/8/1872 in a catholic church in (I think) Port St. Mary. Godparents were John Sullivan (I think an illegitimate child of Frances) and Helen Whelan.

I am looking for a marriage record for Thomas and Frances. I have no idea if Thomas was Manx in origin but I think Frances was born in Ireland.

By 1881 census Mary Hudson and John Sullivan were both in Liverpool but I can find no trace of Frances.

Can anybody out there help me.