Exploring Ireland with your canine companion can be an incredibly special experience, and of course I would be more than happy to help you with your questions, so that you can have a wonderful time on your holiday.
To get straight your question, I am happy to tell you that in principle dogs are allowed to swim in public waters, as long as there are no local signposts declaring otherwise. While you are out and about with your dog in Ireland, please keep an eye on local signposts that share local regulations on leashes, permissions to swim (for people as well as their dogs) and whether dogs are allowed into those particular areas. Please bear in mind, that if an area is not safe for people to swim, then dogs shouldn't swim there either.
To help you select the best Irish location for your holiday, I have a website here called The Beach Guide
; it lists everything there is to know about Irish coastal beaches along with dog regulations for each one.
I have also gathered a bunch of other (hopefully helpful) information for you, to help you plan your trip. First I would like to share a link to our own article on the subject: http://www.ireland.com/en-gb/about-ireland/must-know-information/bringing-your-pet-to-ireland/
Depending on your country of residence there are different regulations for bringing pets into Ireland. Please take a look at the citizens information guide on traveling with pets, to find the specific regulations that would apply to your canine companion.
It's also good to note that Ferry companies and Airlines, each have their own regulations about traveling with pets, and so you should contact your chosen carrier prior to booking, to determine their individual requirements.
Finally, I have a list of websites for you here, that list pet friendly accommodations in Ireland:
B&B Ireland https://www.bandbireland.com/themed-holidays/pet-friendly-homes
Bring Fido https://www.bringfido.com/
Imagine Ireland https://www.imagineireland.com/dog-friendly-holiday-cottages-in-ireland