"The mug of brown ale" is a traditional jig in A Dorian, built in two parts of 8 bars repeated twice. The melody is played with capo 5th.
As you can see in the melody, there is a F# note, which means that we are in Dorian mode. So for the next grid we can add a D modal/major in the second and sixth bars, instead of the Am, in the A. In the B part we can change a G chord by his relative minor, an Em and play some modal position for the Am.
THE MUG OF BROWN ALE (jig)
You can listen to "The mug of brown ale" in the album "Tuned up" by Brendan Mulholland, Brendan Henry and Paul McSherry, recorded in 2008.
Download the summary (tab + chords)