There are 3 townlands named Magheragart in the Dromore area. One is known simply as Magheragart, the other as Magheragart/Sessioaghs and the 3rd as Magheragart (Donnell). There are Gorman households in two of them. Indeed Gorman is a fairly common surname in that area and care will need to be taken to ensure you follow the right family.
You mention an Ellen who remained in Ireland, around 1900. Is this her here with her parents and siblings:
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Dromore/Magheragart/1741606/
There were 3 other Gorman households in the townland and so they may well all be related:
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Dromore/Magheragart/
James & Ellen Gorman married around 1871 and 13 children, of whom 12 were alive in 1911. Only 9 children remain at home by 1911 including Ellen, which fits with your information. She was a teacher so likely to be a letter writer.
http://www.census.nationalarchives.ie/pages/1911/Tyrone/Dromore/Magheragart/870318/
I found these probate abstracts for Gormans from Magheragart on the PRONI wills site:
Gorman James of Magheragart Dromore county Tyrone farmer died 8 November 1915 Administration (with Will) Londonderry 18 February to Daniel Gorman farmer. Effects £27.D.B.N. DERRY 15.7.82
Gorman James of Magheragart (Donnell) Dromore county Tyrone farmer died 16 May 1919 Probate Londonderry 17 February to Magdalene O'Connor married woman. Effects £3.
Gorman Michael of Magheragart Dromore county Tyrone farmer died 2 February 1914 Administration Londonderry 14 January 1944 to Michael Gorman grocer. Effects £25.
Gorman Patrick of Magheragart Dromore county Tyrone farmer died 28 February 1937 Administration Londonderry 14 May to Annie Gorman spinster. Effects £352 6s. 2d.
Only these abstracts are on-line. If you want to read the full files and wills, then you need to go to PRONI in Belfast and look them up. The last one died intestate so there won’t be a will for it, but the probate file should still contain some information about next of kin.
Access to births within the past 100 years, marriages within the past 75 and deaths within the past 50 years, in what is now Northern Ireland, is restricted and you can’t look them up on-line. You need to go into PRONI or GRONI in Belfast and look them up there. (Or get someone else to do that for you). They are the only two locations where recent data is publicly available.