It lúaithidir...

It lúaithidir gaíth n-erraig.
... roth mbúaile.
... boicshimin ar lár srotha.

(they are) (as fast as) (wind) (of spring)
... (mill-wheel)
... (rush) (on) (middle) (of stream)

Three similies for speed:

They are as fast as a spring wind.
... as a mill wheel.
... as a rush in the middle of a stream.

The first image is found in "Serglige Con Culainn" (LU 3809), the second in "Togail Bruidne Da Derga" (1216), and the third in van Hamel's "Compert Con Culainn and Other Stories" (102.18).