Here’s a map I found on the internet. It probably passed through several websites and I can find no reference for it. Style and content-wise it could be from anywhere between 1650 and 1750. The sea monster is a medieval throw back. You don’t see to many Irish maps of the ‘Here Be Monsters’ sort. Ireland was not getting mapped much when monsters were populating cartography. Monks of Early Christian Ireland were writing down stories and lore, drawing monsters in the margins of those texts, not making maps.
The text in the lower left indicate the map was produced in Holland.
Perhaps calling it a monster is a bit harsh, it seems harmless enough. Could it be based on a whale? Whales are still regularly sighted in the channel between here and Scotland.