The Filid were druidic bards, attached initially to the courts of the Irish tribal kings, who were called the "Rig" in their Celtic communities. Each Rig had an 'honour price' in a legal system where the weight of testimony depended on the witness's aristocratic pedigree. In applying the ancient law of the Celts, the "Brehon" code, a Rig's Filid recited in court from memory his master's genealogical origins to ensure that the Rig's testimony would take priority over that of any of his subjects. 

The Filid also entertained their kings' guests with poetry and with tales rich in moral content on the winter nights between Samain (1st November) and Beltain (1st May.) These poets were clearly able to perform remarkable feats of memory in a society where nothing was written.

By the second half of the 7th century, most of the Filid had converted to Christianity. St. Patrick's first learned convert had been the 5th century Filid, Dubthach. Patrick cleverly shaped his scriptural teaching, based on Roman Canon Law, to the Celtic traditions of the Brehon code: he progressively involved the talented Filid in the settlement of disputes arising from the complexity of integrating the dictates of Rome with those of these ancient Celtic societies. A century later, a long way up Great Glen to Inverness from his Ionian retreat, St. Columba used similar tactics in converting the Pictish king's, Bridei's, druid, Broichan, to Christianity.

For the following centuries up to the Synod of Cashel in 1101, the Bishops, the Asbuig, of this Celtic society with its tradition of orally recited law, now attempting to integrate Canon Law with the Brehon Code, used the Filid to establish their own honour price in the courts. The honour price of a Bishop, Asbuig, came close to that of a tribal king, Rig.

In 1111, the Synod of Rathbeasail accorded the province of Armagh, which included Dal Riada (the area of Argyll in today's Scotland) a total of 12 sees. During the following critical surname period marked by the Normanization of the Scots, there were arguably 12 FilidAsbuig in the service of the 12 bishops responsible for these dioceses extending from Donegal and Down in Ireland, to Argyll in Scotland. These men, who were to have offspring bearing their names, were conceivably deacons, and not under full orders. 

The Gillespie name in its present form dates arguably from this period with the points of genealogical departure originating in these 12 "FilidAsbuig" deacons from the province of Armagh. The present geographical distribution of the name corresponds well to this area covering Northern Ireland and Southern Scotland and lends credence to this origin. However, it is intriguing to note that, in Northern Ireland, the Gillespie name is more associated with the Presbyterianism of Ulster than with the Catholicism of the Republic, to an extent that could lend credence to the idea of a purely Scots origin. If this were so, then the Gillespie families resident in the Province may well be themselves of Scots extraction, many of their ancestors having followed William of Orange's Scottish army into battle against his Catholic Stewart father-in-law, James II.

As with many Scots names, the origin of the Gillespie name is thus linked to that of a profession, that of the bishop's 'lawyer.' This function of Celtic society had long disappeared by the time the Norman scribes committed the names of the Scottish landowners to the Ragman Roll when each swore allegiance to Edward 1st, except the doughty Wallaces, father and son, and a handful of others. By then, Gregorian reform had gained the upper hand on the Brehon Code, and men of learning were expected to apply Canon Law, and to read and to write, rather than to commit to memory.

From the 13th century, Gillespie is found as a forename given by Scots Celtic aristocracy to male offspring possibly destined for a life of the cloth as the wealthier, ambitious families had a ready eye for church property, although the name was clearly not Norman . Like a small number of other pre-medieval forenames, it would be perpetuated within a family during successive generations, and so: 

• a 13th-century 'Gillespic the Clerich' appears at the origin of the Scottish clan Chattan;
• the 13th century progenitor of the Campbells was Gillespic O Duithne "Cam (crooked) Beul (mouth;)" 
• the 14th century 5th MacEwen of Otter was called Gillespie Mac Eoghain na h-Oitrich; 
• we find one of the Bruce's MP's in Saint Andrews, Gillespie MacLachlan.