"The Naturalists Companion" was written and illustrated by Digby, probably in the mid-1810s, before he left Dublin for England and Cambridge University. His text, which describes animals and objects he encountered in a number of museums in Dublin, is illustrated with 440 separate watercolours. The text describes the collections in the museums of Trinity College (described between pages 115 and 243) and the Dublin Society and a public menagerie exhibiting at the time. He also described specimens from his own observation and other published sources. The manuscript describes and illustrates a variety of animals, fish, insects, natural and ethnographic productions and antiquities from England, Ireland, India, Spain, Africa, China, America, Australia, New Zealand and the Pacific. It includes a number of Australian animals, such as the kangaroo, and illustrations of Pacific artifacts collected on Captain James Cooks second and third voyages held by the Dublin Society and the Trinity College Museum. 

It seems that Digby composed the text himself although much of its content is drawn from published authorities. He cites numerous standard reference sources such as George Buffon's "Natural History". The text, which concentrates mainly on animals (about 300 of the illustrations are natural history) rather than ethnographic or antiquarian specimens, is largely anecdotal. He is not interested in the anatomy or physiology of the animals he describes, and nor does he attempt to position them within a classification scheme. He uses popular rather than scientific names. It is significant to note that living Australian natural history specimens had reached Dublin by the early 1810s as part, it appears, of a commercial menagerie. The Pacific artifacts illustrated in "The Naturalists Companion" were held in both the museums of Trinity College, Dublin the Dublin Society. The artifacts were probably collected on Captain James Cooks Second and Third Voyages. "The Dress of a Chief Mourner, from Otaheite" (page 221), for instance, was collected on Cook's Second voyage by surgeon James Patten, who settled in Dublin immediately after his return. He gave his collections to Trinity College in 1777, which were later transferred to the National Museum of Ireland. The dress itself was presented by the National Museum to the Bishop Museum in Hawaii in 1971, and was exhibited in the Bishop's 1978 exhibition "Artificial Curiosities. An exposition of Native Manufactures" and is illustrated at fig. 211. Other material probably came from Captain James King, of the Third Voyage's Resolution (see J.D. Freeman, The Polynesian Collection of Trinity College, Dublin and the National Museum of Ireland, "Journal of the Polynesian Society", vol. 58, 1948 p.1-18). Other Pacific artifacts, such as "Sandwich Island God" (page 3) and "A knife of the Sandwich Islands" (p.213), were in the Museum of the Dublin Society, although how they came into the possession of that Society is not known. It was transferred to the National Museum of Ireland, along with much of the Society's collection in 1880. The original was exhibited at the Bishop Museums (Honolulu) 1979 exhibition "Eleven Gods Assembled", curated by Dr Adrienne Kaeppler.

"The Naturalists Companion" is an apparently random compendium of natural history, ethnographic and antiquarian specimens. Its lack of obvious taxonomy indeed its unusual juxtapositions (page 5, for example, depicts a Non Pareil Parrot, a Pied Butcherbird and a Hooka Pipe) illustrates a then common approach to the description of the natural world: an encyclopedic record without an obvious system or organising principal. "The Naturalists Companion" reflects the often haphazard composition of late eighteenth century and early nineteenth century European museums. Comparisons with published catalogues from such museums, such as the Leverian Museum or William Bullocks Museum shows how close Digby's manuscript was in conception to contemporary museums -- see for example William Bullock, "A Companion to the London Museum and Pantherion", 1813 (Mitchell Library call no. 507/B) or King & Lochee, "Catalogue of the Leverian Museum", 1806 (Mitchell Library call no. 570.7/L).