The shirt was worn as a man's undergarment, covering the body from neck to knee

[Puffer]

The shirt was worn as a man's undergarment, covering the body from neck to knee. Most were made of white linen which could be very fine or very coarse.  A laborer's shirt was usually made of unbleached linen (cotton was $$$) or small patterned checks and stripes. A plain shirt usually served as a nightshirt + worn every day ! ( + Seldom washed  ) {remember most had only 1 set}

[doggoner]

Puffer

Rather like a ladies chemise???

doggoner

[rollingb]

All my linen shirts are knee-length. 

[Puffer]

If Wear a kilt this type of garment may have been something wore 

{Léine The léine or shirt can be considered the mainstay of Irish and early Scottish clothing. It was worn from mid thigh to below the knee depending on if it was worn alone or with trews. Fashioned of linen, the léine was dyed a saffron color for those of better standing.}  Below is an example of 1 being worn with the  Great Kilt (or Feileadh Mòr )

I often wear 1 with my "small kilt"  Why #1 it helps to keep my kilt clean.(if you have had to clean 1 or had pay to have 1 done, you know why  )  Also ads warmth to a synthetic kilt. + the linen or cotton = more "pleasant" agin my skin than coarse Wool