When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Knock is roughly similar to Donegal. You can assume this to be the case over the next 21 days.
Expect the weather to be around 19 °C on average in July. As the temperature peaks, try to get your plans done early, as flight price and availability will have an impact on your travels. The climate in Donegal is generally colder than in Knock.