John Mill Fortescue

[Sinann]

If you do a search of the Chief Secretary's office for JM Fortescue there are more letters, he appears to be stationed in Sligo, also he returns to England for a while as one of his children is dying.

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Pedigree of Fortescue, of Dromisken, Co. Louth, Birchland Phillipaugh, Co. Devon, of Newrath, Co. Louth, c.1066 -- 1792.   

Dublin: National Library of Ireland, Genealogical Office: Ms.164, pp.170-173

Birchland Phillipaugh  I presume is   Buckland-Filleigh

[Sinann]

He gave his son John the second name Faithful the same as this Faithful Fortescue https://en.m.wikipedia.org/wiki/Faithful_Fortescue
So perhaps Mill comes from an earlier relative as well.

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Pedigree of Fortescue, of Dromisken, Co. Louth, Birchland Phillipaugh, Co. Devon, of Newrath, Co. Louth, c.1066 -- 1792.   

Dublin: National Library of Ireland, Genealogical Office: Ms.164, pp.170-173

Birchland Phillipaugh  I presume is   Buckland-Filleigh

You can email NLI and ask cost of Pedigree which you can then use a Source for everyone on it.
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A Genealogical and Heraldic Dictionary of the Peerage and Baronetage... 

http://www.rootschat.com/links/01mar/