When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Belfast City is roughly less than Donegal. You can assume this to be the case over the next 25 days.
You can imagine temperatures to be in the 19 °C range in July. The weather in Donegal is generally colder than Belfast City's. As flight availability is subject to become more limited as peak season approaches it is ideal to book in a timely manner.