Donald, it would be quite a task to transcribe what Alf plays of 'All For me Grog', because it's pretty complicated and would be quite hard to work out just by ear. The intro is played as single-note melody, then for the verses it's a mixture of chords, melody + chords, and harmony lines. He throws some higher-pitched chords into the second chorus as well. A nice example of his accompaniment at its best, IMO.

If you're a relative beginner, I'd suggest working out a few basic chords on your instrument and using those to accompany your singing with rhythmic strikes on the beat. Lloyd sings 'Grog' in F and the chord sequence is something like:

[F] All for me grog, me [Bb] jolly, jolly [F] grog
[F] All for me beer and to- [C] -bacco
[C] For we've [F] spent all our tin on the [Bb] lasses drinking [F] gin
And across the Western [C] ocean we must wan- [F] -der.

Personally I don't enjoy Alf's accompaniment to 'John Barleycorn' nearly as much. He plays nothing but D minor chords (stressing the first three beats of each bar) on lines 1,2 and 4 of each verse, which sounds quite dull and discordant to my ears. He does something more interesting and fluid on line 3, though.

If it were me I'd probably chord it something like this:

There [G] was three [Dm] men came out of the West, their [G] fortunes for to [Dm] try
And [G] these three [Dm] men made a solemn vow John [G] Barleycorn should [Dm] die
They [F] ploughed, they [Am] sowed, they [Dm] harrowed him in, throwed [F] clods u- [G] -pon his [Am] head
And [G] these three [Dm] men made a solemn vow John [G] Barleycorn was [Dm] dead.

Actually I'd probably not use minor chords at all, but use simple 1+5 chords throughout.

Hope that's helpful.