The original words were indeed in Gaelic, but the better-known version for non-Gaelic speakers is the lyric by Sir Hugh Robertson, as follows:

Step we gaily, on we go,
Heel for heel and toe for toe,
Arm and arm and row on row,
All for Mairi's wedding.

Over hill-ways up and down,
Myrtle green and bracken brown,
Past the shielings, through the town;
All for sake o' Mairi.

Red her cheeks as rowans are,
Bright her eye as any star,
Fairest o' them a' by far,
Is our darling Mairi.

Plenty herring, plenty meal,
Plenty peat to fill her creel,
Plenty bonnie bairns as weel;
That's the toast for Mairi.

The original song was written for a Mary (to celebrate her winning a national song contest), and most of the versions I've heard sung in English have pronounced Mairi as "Mary".