Okay, Cathal wasn't assuming or misreading, this was definitely for his benefit, and he couldn't deny he was appreciating it. (Even if he also knew it was completely unnecessary. He'd already been interested before she ramped it up.)
He wanted to lean over towards her and say I like those jeans on you but Marcie's dad was right there, and so he kept his mouth zipped on the tight jeans front. Instead he said, "those clothes look too nice to get grease on. Are you going out?" (Because, shit, that was also possible, wasn't it. She was looking hot momentarily around him because she was on her way somewhere. He had to stop assuming!)