The lyrics posted by Dave above seem to be a faithful transcription of McGinn's recording; however the text posted at Matt McGinn's web site has a few extra verses (besides different spelling):

ROB ROY MCGREGOR

1. Who's the cause o' a' the mirth?
Rob Roy MacGreegor-O
Frae Glasgow Cross tae the Solway Firth
Rob Roy MacGreegor-O

CHORUS: Eat the breid an' heid the ba'
The man that ate the bile't ham raw
He very seldom kicks his maw
Rob Roy MacGreegor-O

2. On a dainty sandwich he's gey keen
Two loaves o' breid an' a pig between

3. When he's tae the dancin' gane
He aye sees fourteen lassies hame

4. Who puts on size fifteen shoes?
Drinks a barrel o' the rye-buck booze?

5. Who will right Auld Scotia's wrongs?
Who is the leader o' the Tongs?

6. He never ever sings a song
Unless it's a hundred verses long

7. That wis a real wee rotten verse
This one here is ten times worse

8. At London toon he had a look
Kiss'd the Queen and chased the Duke

9. Frae Partick Cross up tae the Tron
Who knows every pub an' pawn?

[From "Matt McGinn Again" (1967) and "The Best of Matt McGinn" (2001).]