Yes - birthdate was totally different on the 1939 register. Where did you get your Ellen's full birthdate from?
A couple of things bother me.
There is no marriage in England between a James O'Brien & Ellen Furlong to definitely rule them out as the wrong couple.
The cwgc entry shows James parents were in Waterford which is where you say your Ellen & James married in 1915.
They were only married 2 months when he was killed & there are no O'Brien births with mmn Furlong which supports a short marriage.
If he was serving in an Irish regiment when they married - why did they move to Bootle?
The 1921 entry will have her birthplace & age. If it shows Waterford you cannot dismiss the 1916 marriage without buying a copy of the cert to establish her fathers name. Unfortunately it was a civil marriage so can't be looked up in parish records