I have the same buttons on my Argyll jacket. It is Scottish Gaelic written in the Old Irish alphabet, and it's a bit difficult to make out. Whether this is from the moulding or not, it is hard to distinguish the letters "i" or "l".

Mine reads:
Clann nan Gael (Family or tribe of Gaels)
An Grailib (Grallib, Grailibh?) na Creile (or Crelle?) (The ? of the ?)

There are definitely no fada (length marks/accents) on the buttons. As this is in the Old Script, it would have been represented by a straight line above a vowel, not like the acute (Irish) or grave (Scottish) accents used today.