The first documentary mention with an exact date comes from 1256; it is a document under whose terms Archbishop Arnold II of Isenburg approved the transfer of bondsmen from Ilriche to Himmerod Abbey.

Towards the end of the 18th century, the tithes from Illerich were the subject of disagreement between the Countess of Sternberg and the Abbot of Brauweiler, leading to a rather lengthy court case between 1783 and 1787.

[5][6] The German blazon reads: Das Wappen der Ortsgemeinde Illerich ist über einem silbernen Schildfuß, in dem zwei rote Raben enthalten sind, gespalten von Rot und Silber.

The municipality's arms might in English heraldic language be described thus: Per pale argent a cross gules surmounted by a rose Or and vert an ear of wheat couped of the third and a scythe blade palewise, the point to base, of the first, in a base of the first two ravens, wings endorsed, of the second, armed sable, the one to dexter sinister.

The version shown at the municipality's website shows the cross “enhanced”, that is, with the crossbeam slightly raised above centre.

Illerich grew up in early times from an estate to today's municipality, in which agriculture has been practised to the present day.