It is correct that to a certain extent the IPP were liable for their own downfall in the period 1914-1918. This was due to their style of politics being outdated, out of touch with the Irish nationalist public opinion, lack of co-ordination within the party and the aging party leadership. However, other factors must be considered which contributed to the IPP's demise. The Easter Rising of 1916 and how the British government dealt with this reflected badly on the constitutionalist IPP. They relied heavily on the British government to deliver Home Rule, but the lack of consistency and poor decision making was exposed during this time. Inevitably, the Irish public grew to distrust the IPP to some extent, and with the increasingly violent mood in Ireland, the people began to search for a stronger solution. This came in the shape of the Sinn Fein party, a dedicated band of nationalists who were co-ordinated, clear about what they wanted, and were not associated with the British government. In fact, the British government labelling the Easter Rising as a Sinn Fein stunt contributed to their publicity.
Therefore, it can be argued that the mistakes of the IPP was a prominent factor which led to their downfall, however an historian must acknowledge other factors of the time to answer this question fully.