Merry this is a big guess here, I'd calculate 1805 for Thomas.
From the documents I acquired at the Sligo Heritage and Genealogy Centre - His son John married 23 Feb 1866 RC Chapel of Bunninaden, Parish Kilshalvey, Kilturra (Ballymote) to Bridget Finn. He was a widower aged 32 years. a farmer of Gurtagara. Makes John born 1834 ish, on the marriage cert his father's name was Tom a farmer (which ties in with John's son Michael being known as Michael John Tom - oodles of McDonagh's around Gurteen and another McDonagh family in the townland of Gurtigara/Gurtygara in late 1890s). I estimate Tom/Thomas married around 1830, born 1805.
On Griffith Valuations 1858 Gortygara John McDonagh Senior and Junior lease from Lord De Freyne - Land, House & Offices (outhouses for animals). Mary McDonagh leased a house from John Jnr.?