The actual marriage record is available on GRONI's website for a small fee but I just used several indexes to get the details shown. The certificate should list their residences and fathers' names & occupations which might allow you to find further records of the family in Fermanagh (although with a surname like Johnston this may be difficult).
Just a bit more for you to follow up on after re-checking Belfast cemetery database, PRONI will extracts, etc.-
Edwin John Johnston (c1864-2 Feb.1929) m.(1889 Enniskillen dist.) Eliza Ellen Nixon (c1861-13 Apr.1907)
1907- 12 Outram St.; 1911- 10 Outram St.; 1929- 36 Outram St.
1. William Robert (6 Nov.1889) m.(1919) Margaret Ann (d.1922) ?m.2 Sarah
2. Elizabeth (29 Oct.1890)
3. Edwin James (5 Mar.1892-25 Jan.1948) m.(1914)
4. Isabella Dorothea/Dorothy Mary/May “May” (6 May 1893-13 Aug.1976) m.(22 May 1913 Belfast) Charles Anderson Miller (d.29 Apr.1936)
Miller Charles of 73 Ormeau Road Belfast painter died 29 April 1936 at The Royal Victoria Hospital Belfast Administration Belfast 24 July to May Miller the widow. Effects £436 5s.
5. Samuel Hugh (4 Apr.1894-19 Jan.1933) m.(1929) Mary Jane. 1933- 23 Outram St.
6. Rebecca Ellen “Nellie” (10 Jan.1896-18 Oct.1941) m. James Swann (c1896-11 June 1970)
Dundonald Cemetery (also 1 other burial in plot)
Margaret Ann Johnston, age 27, died 25 Mar.1922
William Robert Johnston, age 50, died 9 Feb.1940
Sarah Johnston, age 73, died 11 Apr.1959
Johnston William Robert of 126 Ulsterville Avenue Belfast labourer died 9 February 1940 Administration Belfast 11 March to Sarah Johnston the widow. Effects £165 1s. 5d.
Belfast City Cemetery- Glenalina Extension
James Edgeworth Johnston, age 3 3/12, died 4 Nov.1920
Edwin James Johnston, age 55, died 25 Jan.1946
Mary A. Johnston, age 82, died 3 Jan.1975
Belfast City Cemetery
Charles Anderson Miller, age 46, died 29 Apr.1936
Isabella Dorothea May Miller, age 83, died 13 Aug.1976
Dundonald Cemetery
Rebecca Ellen Swan, age 62, died 18 Oct.1941
James Swann, age 73, died 11 June 1970
William R. Swan, age 62m died 28 Feb.1987
James Swan, age 65, died 18 Oct.1991