Oriel shook his head with a smile. Most people were surprised to discover that. "They're not," he replied. "Irish Sign Language is actually closer to French Sign Language than to British."
He waved his hand in the air in a 'sort of' gesture that needed no verbal explaination. "Not really. But there are a lot of Magical Words that don't exist in any Muggle Sign Languages." Obviously. "And there are... dialect variations." The same way that English could vary depending on where in the world it was being spoke.
"I think because the Wizarding population is relatively small, they just adopted Muggle Sign Languages - the same way we speak the same verbal languages?" Or so he would guess.