Complex sentence: The slow worm is not native to Ireland, but is believed to have been illegally introduced in the late 20th century, though it has only been sighted in parts of County Clare, mainly in the Burren region.
Split into simpler sentences: The slow worm is not native to Ireland, but is believed to have been illegally introduced in the late 20th century. Though it has only been sighted in parts of County Clare, mainly in the Burren region.