"Tha mac luath agam."

Translation: I have a fast son.

[2CGV1uMG]

To remember "mac" think of MacDonald meaning son of Donald or MacAdam meaning son of Adam.

[ScottishScones]

could luath also be used to mean clever?

[PatriciaJH]

I wondered about that too, but I don't think so, because it's not one of the meanings given at either Am Faclair Beag or at Wiktionary. It just seems to mean fast, quick, swift, or early, soon.
https://www.faclair.com
https://en.m.wiktionary.org/wiki/luath