The Filid were druidic bards, attached initially to the courts of the Irish tribal kings, known as the Rig. The Rig had an honour price in a legal system where the weight of testimony depended on the witness's aristocratic pedigree. In applying the Celts' ancient law, known as the Brehon code, the Rig needed his Filid to recite his genealogical origins in court to ensure that his testimony would take priority over that of any one 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 delivering their kingly panegyrics.

By the second half of the 7th century, 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 to the Celtic traditions of the Brehon code, and progressively involved the talented Filid in the settlement of disputes arising from the complexity of integrating Church Canon Law into the Brehon code. A century later, having journeyed the Scottish Great Glen to Inverness, St. Columba used similar tactics by converting the Pictish king's, Bridei's, druid, Broichan, to the Christian faith.

For the following centuries up to the Synod of Cashel in 1101, the Bishops, the Asbuig, of this Celtic tradition of orally recited law integrating both Canon Law and 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 Argyle in modern-day Scotland) a total of 12 sees. During the following critical surname period marked by the Normanization of the Scots, there were arguably 12 FiliAsbuig in the services of the 12 bishops responsible for dioceses from Donegal and Down in Ireland to Argyle in Scotland. These men were possibly 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.

As with many Scots names, the origin of the Gillespie name is linked to that of a profession, essentially that of the bishop's lawyer. This function of Celtic society had disappeared by the time the Norman scribes committed the names of the Scottish aristocracy to paper in the spirit of the 1296 Ragman Roll. Gregorian reform had gained the upper hand on the Brehon Code, and now men of learning were expected to read and write rather than to commit to memory.

The name was used briefly as a forename by aristocracy with male offspring destined for a life of the cloth. The wealthier, ambitious families had a discerning eye for church property. Like a small number of other pre-medieval fore names, it would be perpetuated during successive generations in the families concerned if the cleric had offspring.

A Gillespie 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 5th MacEwen of Otter in the 14th century was called Gillespie Mac Eoghain nah-Oitrich; we find one of the Bruce's MP's in Saint Andrews, Gillespie MacLachlan.

The Norman scribes translated the names resulting from the tribal kings' poets, FildRig, predictably, to Gilroy, as the French for king is 'roy' or 'roi', or to Gilry.

There are notions concerning the etymology of the Gillespie name which are no longer credible. The first associates the root with "Gilly," a serving boy. If this were true, the Scots Gillivray name, where the the third syllable comes from the Gaelic "Brath," meaning judgment, would foolishly translate to, 'servant boy of judgment,' rather than to the obvious FiliBrath, or 'lawyer (poetic reciter) of judgment.' The Galbraith name has the same origin. Another myth links Gillespie to the name to Archibald: it is absurd to link the Gaelic name of Gillespie with the Germanic Ercenbald.