What is the ruling on praying in a mosque surrounded by a cemetery from the outside, and is it obligatory to repeat the prayers that were performed therein if they were not valid?

There is no harm in praying in the aforementioned mosque as long as there is no grave inside it, and there is a barrier between it and the graveyard, such as the mosque's wall, or the street, or a wide distance. The mosque's wall is sufficient as a separator between it and the graveyard, and the presence of graves to the right or left of the worshiper does not cause any harm.