I found several versions at various websites, but none of them seemed particularly authoritative.
Lyrics below copied from http://jackytappet.tripod.com/songs.html#good
[with variants from other sources shown in brackets.]

GOOD COMPANION
[also known as "TO BE A GOOD COMPANION" or "I'LL DRINK ONE" or "THE SUSSEX TOAST"]

I'll drink one if you'll drink two,
And here's a lad that'll drink with you,

CHORUS: And if you do as I have done,
You'll be a good companion.
Companion, companion,
You'll be a good companion.
Companion, companion,
You'll be a good companion.

I'll drink two if you'll drink three,
And here's a lad that'll drink with thee. [or "with me" or "for free"]

I'll drink three if you'll drink four,
And here's a lad that'll drink some more. [or "for an hour"]

I'll drink four if you'll drink five,
And here's a lad that'll drink till he dies. [or "we're the lads that'll drink till we die"]

I'll drink five if you'll drink six,
And we're the lads that'll drink like bricks.

I'll drink six if you'll drink seven,
And we're the lads that'll drink in heaven. [or "drink eleven"]

I'll drink seven if you'll drink eight,
And here's a lad that'll drink till he's tight. [or "till it's late"]

I'll drink eight if you'll drink nine,
And we're the lads that'll drink anytime.

I'll drink nine if you'll drink ten,
And we're the lads that'll drink again.