Griffiths Valuation for 1860 lists William Blythe farming on plot 2 in Tamlaght. That was a farmhouse, outbuildings and 27 acres. Next door on plot 1c was Thomas Blythe with a dispensary, house and small garden, so it appears he was a in charge of the local dispensary. He seems likely to be related to William.
http://www.askaboutireland.ie/griffith-valuation/index.xml
William?s farm today is on the Springhill Rd, a minor road off the A5 Curr Rd, between Ballygawley and Omagh.
The revaluation records show William Blythe left or died in 1865. He was replaced by John Wilson.
http://www.proni.gov.uk/index/search_the_archives/val12b.htm
The GRONI site has a death for a William Blythe regd in Clogher on 11.5.1871 aged 81. You might want to view it to see if it was your ancestor.
You can view the certificates on-line on the GRONI website, using the ?search registrations? option:
https://geni.nidirect.gov.uk
(You will need to open an account and buy some credits. It costs ?2 to a view a certificate).
The revaluation show Thomas Blythe?s name remaining against plot 1c until 1906 when he is replaced by William G Blythe. This seems to be Wm G in the 1911 census:
http://www.census.nationalarchives.ie/pages/1911/Tyrone/Cecil/Tamlaght/855717/
There?s a death for Thomas Blythe registered in Clogher 30th June 1905 aged 83. So his age might make him a brother to your William Blythe who married Anne Marshall.
Regarding the Marshall?s townland, I wonder if it was Kilnaheery. There were two Marshall farms listed there in 1860. Robert on plot 15 with 32 acres and Mary (likely a widow) on plot 23a with 42 acres.
1901 census:
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Cecil/Kilnakeery/1725847/
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Cecil/Kilnakeery/1725848/
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Cecil/Kilnakeery/1725856/
Some probate abstracts (summaries for these families):
Letters of Administration of the personal estate of Patrick Marshall late of Curr County Tyrone Farmer who died 24 December 1885 at same place were granted at Londonderry to Thomas Marshall of Kilnaheery in said County Farmer the Son.
Letters of Administration (with the Will annexed) of the personal estate of Robert Marshall late of Kilnaheery County Tyrone Farmer who died 27 April 1886 at same place were granted at Armagh to Thomas Marshall of Kilheery Farmer a Brother.
Marshall Thomas of Kilnaheery county Tyrone retired farmer died 22 June 1904 Administration (limited Grant) Londonderry 23 December to Joseph Raulston auctioneer. Effects ?1.
Clogher Church of Ireland records start in the 1760s. You probably would need to go through those to trace Anne & William?s baptisms, siblings etc. They don?t appear to be on-line anywhere. However there is a copy in PRONI (the public record office) in Belfast. A personal visit is required to access them.