Such a marriage would be Inter-denominational. The Catholic Church has long viewed marriage with people it calls heretics as illicit without a Bishop's dispensation, as promulgated by the Pio-Benedictine Code of 1917. Prior to this date there was some regional variation and in interpretation and practice, which the Pio-Benedictine Code harmonized.
So my view is that either Kathleen was incorrect about the 15 years, or that the Church in different parts of the world adopted the Pio-Benedictine Code with differing levels of haste, perhaps due to local considerations. In any case, Kathleen was probably incorrect about the case in her "own country", since the Canon Law has stood for centuries (up to a millennium) and had been made more explicit by the Ne Temere decree of 1907 and the Pio-Benedictine Code of 1917.
Certainly, according to the Pio-Benedictine Code, the dispensation could have been given, but was it was unlikely. The wikipedia article mentions "just and reasonable cause", but in practice the interpretation was that only extreme cases were considered. Just being in love was (and still isn't) enough.
But to answer your question, the rules didn't really change, and they were not explicitly designed to "dissuade Catholics from marrying protestants". The rules were about trying to ensure that any children of the union were brought up in the Catholic faith.