Hi Michelle,
Donegal is a hot spot for basking sharks and whale watching, we have a dedicated
article on the topic which you may find of interest. Donegal is a short journey from the North West Coast of Northern Ireland if you wanted to make the journey. Companies like Aquaholics provide boat tours from Portstewart and Portrush if you are not looking to venture to Donegal.
I hope this helps, let me know if you have ay other questions.
Cathaoir