Mary, Mother of all Saints, pray for your children in America. Pray for those who seek to imitate your example in humble service to the Gospel of Life and the poorest of the poor. Pray that your son may lead us from the darkness of sin to the light of his eternal glory.
-- Novena to Our Lady of Guadalupe, Queen and Mother of America.
If you're in church when you hear this, they mean the Blessed Virgin Mary, Mother of Christ. This goes for all holy books, hymnals, prayers, religious incantations, etc.
If, on the other hand, you find yourself amongst the lower classes of 19th century England, you will more likely hear this phrase used in reference to the miraculous pitcher, the niche-cock, the nonesuch, or to put it bluntly, the vagina.
Obscene and blasphemous! Back in the 1800s, this was one of the more common euphemisms for female genitalia. Along with mother of all souls and mother of St. Patrick, these were technically indisputable, contained no dirty words, and were highly amusing -- the perfect combination. They were in common use in both England and Ireland, and probably in other areas as well, although I can't find any reference to American usage.
The 1811 Dictionary of the Vulgar Tongue goes so far as to define 'the mother of all saints' as the Monosyllable, but that's a bit of an exaggeration. TMoaS is a joking and humorous reference, crude, but not that crude.
It's worth noting that I could not find a written text using the phrase 'the mother of all saints', aside from dictionaries. Apparently it was used almost exclusively in spoken language, and rarely if ever written.