When you look at the distribution of baptismal and marriage records over the years for the surname Skeath (and Skeith), you don't get a sense of this being a family widespread in / indigenous to Co Monaghan before the 1800s (if any of us can call ourselves indigenous to anywhere!).
Baptisms: 1820s-30s mainly Co Armagh; 1840s-50s mainly Co Antrim but including 2 in Co Monaghan; 1860s-80s mainly Co Armagh, 1890s mainly Co Antrim.
Marriages: 1830s-40s a couple in Co Down and 1 in Co Monaghan; 1850s-60s mainly Co Armagh; 1870s a big cluster of them in Co Down; 1880s-90s mainly Co Armagh; 1900s mainly Co Down.
Not terribly scientific I know, especially given the proximity of Armagh and Monaghan, but interesting all the same.
More interesting was having a punt by looking at the earliest Co Monaghan marriage I could find, for a Richard Skeath to Honor McElloon in 1835 and while she's from Monaghan, his address is given as Tullylish Dio. Dromore.
So for what it's worth, that would seem to be one connection between the Tullylish Skeaths and the Skeaths in Co Monaghan.