As far as the structure of the Celtic caste system is concerned it would appear that it closely followed the original Aryan one:
Irish caste system
Druids (including Ovates and Bards)-priestly caste
Bo aire-"cow herds"-producer caste
Gallic caste system
I would like to make two points here. Firstly my readers will note the use of the term aire which has exactly the same meaning as the term Arya or Aryan. This puts paid to the lie of many 'academics' that our European ancestors did not use this term to describe themselves. Peter Berresford Ellis writing in The Ancient World of the Celts compares certain Old Irish and Sanskrit terms. The Sanskrit Arya is translated as 'freeman' and the Old Irish aire as 'noble'. In the Germanic caste system the noble was the Jarl caste whilst the freeman was the Karl caste, the yeoman (or artisan) in other words. I have proved before this term is not limited to the Iranians or Indo-Aryans. See http://aryan-myth-and-metahistory.blogspot.co.uk/2008/12/aryan-term-not-confined-to-indo.html Indeed there is also a close connection between the Aryans and agriculture. The Aryan was not just a warrior but a farmer and he revolutionised agriculture by the invention of the plough. The English yeoman of the late Middle Ages is the epitome of this. See http://aryan-myth-and-metahistory.blogspot.co.uk/2014/06/the-aryans-and-farmer-caste.html The second point that I wish to make is that the above mentioned caste system is typical of Ireland but Caesar writing in his De bello Gallica stresses the importance of the two highest castes:
“In the whole of Gaul two types of men are counted as being of worth and distinction. The ordinary people are considered almost as slaves: they dare do nothing on their own account and are not called to counsels.“Of the two types of men of distinction, however, the first is made up of the druids, and the other of the knights.” (Book 6.13, Carolyn Hammond translation)
If Caesar is correct in his analysis then this may be an indication that the Celtic tribes of Gaul were more warlike due to the unsettled nature of the times with the threat that they faced from Rome. This situation would have increased the importance of the warrior caste whilst Ireland was largely free from conflict from external enemies and so the dominance of the two highest castes was not so evident. It would appear that there was a fair degree of mobility between the priestly and warrior castes of the Gauls but less so between these two castes and the third caste although it was also possible to rise from this caste to either of the other two but it took several generations for this position to be consolidated and to be fully recognised.
The term bo aire literally means cattle chief and cattle were regarded as an indication of wealth in both Celtic and Germanic societies and this is of course reflected in the Fehu/Feoh/Fe rune, meaning cattle or livestock as a form of mobile wealth. Eventually land became an indicator of wealth as the tribes became more settled. This was of course before the introduction of 'money'.
The ancient Brehon Laws of Ireland date back to the Iron Age and they developed from oral laws as did the Germanic legal systems. It is said that they have their foundation in Proto-Indo-European or Aryan times. The Brehons were judges or arbiters of the law. The Brehon Laws describe the structure of Irish society and show that there were five main classes of people:
Kings of various grades from tribal Kings to the High King.
Nobles (which included Kings)
Non-Noble Freemen with property
Non-Noble Freemen without property or with little property
The first three classes were known as 'privileged' and known as an aire. The nobility held land which they owned and thus were the aristocracy. Another term for this type of noble or chief was Flaith. The Freeman with property although not classed as a noble was nevertheless an aire. This would seem to equate to both the gentry and the yeomanry of late mediaeval England. The bo aire was the equivalent of the English yeoman or franklin as I have said previously. A wealthier bo aire could rise to the lowest rank of noble. The Freeman with little or no property were termed ceile or producers. The Freeman whether he be the owner of property or not would have equated to the Karl caste of the Germanic system. Craftsmen or artisans were also regarded as Freemen of the lower rank. The Non-Free were not all slaves and some could own small plots of land for subsistence purposes and they would have equated to the Thrall caste of the Germanic system. The Druids are not represented in the structure outlined in the Brehon Laws as this concerned purely the structure of lay society, not priestly.
It is clear that the Kings and Nobles equate to the Jarl caste whilst the Freemen of both classes equate to the Karl caste and the Non-Free the Thrall caste, thus reconciling the system in the Brehon Laws to the tripartite Indo-European system. If one considers that the Non-Free as a type of Sudra caste fall outside of the Indo-European tripartite structure then what we have is as follows:
Nobles (including Kings)-warrior caste
Freemen (farmers, craftsmen with or without property)
Non-Free-slaves and others of low standing falling outside of the aire and equate to the Indian Sudras.
The Germanic system does not have a separate priestly caste as this function was subsumed by the noble caste and we see the two functions of the Jarl caste divided into soverignity (Tyr) and magic-religion (Odin) and thus the Thralls which would not have been regarded as aire by the Irish are formed into the third caste, the other two moving up one level to replace the lost priestly one. I have already discussed the reason for this in http://celto-germanic.blogspot.co.uk/2014/04/the-germanic-caste-system-reappraisal.html