A field system, in archaeological terms, refers to the surviving physical traces of former land division: walls, banks, ditches, or lynchets (the stepped ridges that form on slopes when soil accumulates against a boundary over generations of ploughing) that together reveal how people once organised and worked the land. These features can range in date from the Bronze Age through to the post-medieval period, and in the west of Ireland they are often associated with communities whose farming practices were interrupted or erased by clearance, famine, or simple abandonment. What remains at Troscaigh Thiar is recorded as a monument, meaning it has been identified and designated as part of the archaeological heritage of the state.

The source material available for this particular site is currently limited, and no specific dates, associated finds, or detailed descriptions can be drawn upon here. What can be said is that field systems of this kind, particularly in Connacht, frequently preserve evidence of pre-Famine or even earlier land use, sometimes overlying much older boundaries that were reused across centuries. The townland name itself, Troscaigh Thiar, suggests a western portion of a place, with the Irish word thiar meaning west, a naming convention common across the region where townlands were divided into eastern and western parts to reflect patterns of settlement or landholding.