Lennon included his song Ireland for the Irish (I believe it was called) on one of his solo albums. That, along with his own Irish heritage, would make his support for the IRA unsurprising. I also believe that John, although a highly intelligent individual, was very much the romantic idealist. He often waded in with people and causes based on trust and faith, and was sometimes taken advantage of. I'm sure this was the case with the IRA. He may have been persuaded that his contributions would have a positive impact on the political process of revolution. I don't think John would knowingly have provided funds for guns and bombs.