Proposal for the Union Flag of the second United Kingdom (after the unification of Ireland and Great Britain on 1 January 1801) printed in the February 1801 Gentleman's Magazine as an illustration to a letter dated 3 January 1801 from T[homas] Walters. See plate II, figure 2 in volume LXXI, part 1, pages 121 &amp; 122.
Having in the younger part of my life superintended the making and furnishing the silk standards for the Royal Family, and also the colours for the British navy, my fancy led me to conceive, that the new "Union Jack" (as it is called) would have been agreeable to the drawing now inclosed (fig. 2); and I still think the Irish harp, in its own proper field, placed on the centre of the cross of St. George, would have been handsome and appropriate. ...

The illustration's hatching shows that the ambiguous wording of "its own proper field" intends the central disc to represent the heraldic field of the Irish heraldic banner as green, and not blue (as had been the case since the Middle Ages and as remains the arms of Ireland both in the kingdom and the republic). This letter was published, in the same issue, next to another letter dealing with objections to the kingdom's new royal arms. A response, objecting to the use of the harp among two crosses when a third cross was available and suggesting an alternative arrangement of three crosses, appeared in the March 1803 issue (The Gentleman's Magazine, vol. LXXIII, part 1, pp. 217 &amp; 221, pl. III, fig. 11).

F. Edward Hulme, in his The Flags of the World: Their History, Blazonry and Associations, page 54, deals with this suggestion similarly, pointing out that the design violates the rule of tincture, with the gules (red) and vert (green) (two heraldic colours) in contact: ... the flag of the first Union, ... but placed in the centre a large green circle having within it the golden harp of the Emerald Isle: but this is objectionable, as it brings green on red, which is heraldically false, and as Ireland has a cross as well as England and Scotland, it seems more reasonable to keep the whole arrangement in harmony
 and it, ... like all the other suggestions, good, bad, and indifferent, suffered from the fatal objection that it saw the light when the whole matter was already settled and any alteration scarcely possible.

NB that in this design, to admit the green disc and gold harp, the red cross must be quite broad; here its thickness is equal to one quarter the height of the flag, as is used for Union Flags made to the late Victorian "War Office pattern" (e.g. for military colours). The white saltire's thickness is equal to one fifth the height of the flag, as is traditional for saltires in heraldry and is used for Union Flags made to the 19th-century "Admiralty pattern". Here, the combined thickness of the red cross and its border ("fimbriation") is equal to one third the height of the flag, as in the "Admiralty pattern" Union Flag. In the illustration in The Gentleman's Magazine, the red cross is also broad to accommodate the Irish components, but the white saltire is much narrower, as frequently resulted from using the same breadth of cloth for the making of the saltire as for the making of the fimbriations. The green disc is here arbitrarily and anachronistically taken from a modern digital rendering of the Irish tricolour; the other colours are as used in the 21st century British flag. Not to be confused with the similar flag actually in used during the Interregnum, where a harp similarly appeared in the centre of the Union, but on a blue inescutcheon rather than on a green disc.
The Gentleman's Magazine, February 1801 (vol. LXXI, part 1, plate II, figure 2, volume LXXI, part 1, pages 121 &amp; 122).
Own work based on: Arms of Ireland (historical).svg:   and Flag of Great Britain (1707–1800).svg: