When Trevor Howard (priest) goes in search of the distressed Robert Mitchum (schoolteacher) he leaves the schoolhouse carrying a bundle of clothes and the schoolteacher's black leather boots. Later, when he discovers the schoolteacher on a rocky shoreline the priest is still carrying the clothes and boots. However, during the search, the priest is stopped on a beach by British soldiers; at this point the priest is in possession of the clothes but not the boots.
The British army camp outside of the village has several Quonset or possibly Nissan huts. Neither style of hut was developed until early in WW2, about 1941 - 25 years after the setting for this movie.
The Corporal is wearing a 1914/15 Star. It could not have been issued as early as 1916/17.
When Rosy throws the magazine into the surf, a quick shot shows strongly breaking, high waves. Every shot, before and after this one, from every angle shows gently breaking waves no more than a couple of feet high.
The RCA record shown playing is a LP record which was not introduced until decades after the movie is supposed to be portraying.
Father Collins wears a traditional black garment with white "dog collar" but apparently in the period this film was set, the law forbad a catholic priest to dress this way.
As he is driving Major Doryan to the camp, the corporal asks him if he had been in the Second Battle of the Marne. The Second Battle of the Marne was fought in July and August of 1918 near the end of WWI while events in Ryan's Daughter are set in 1916 not long after the Easter Rising.