Never assume a divorce has taken place unless you have proof! Bigamy was more common than divorce for the average person at that time and moving back to Ireland would certainly have helped avoid being caught! I looked up the marriage certificate on GRONI just to confirm my suspicions (see below).
Alfred Charles Harding m.1 (Jan./Mar.1912 Dover, Eng.) Florence Mary Sherwood Brice according to registration index which also shows 4 children born (father Harding, mother Brice)-
1. Albert C.J. Harding (July/Sept.1912 Dover-Jan./Mar.1919 Dover)
2. Doris M.F. Harding (Jan./Mar.1914 Dover-Apr./June 1927 Dover)
3. Irene Anne M. Harding (2 Jan.1920 Dover-Dec.2003 Kent) m.(Jan./Mar.1945 Dover) Frank Firth
4. daughter (Apr./June 1921 Dover)
Alfred Charles Harding m.2 (15 Dec.1919 Belfast) Abigail Halliday (Lyttle) Duff
1. Henry Stewart Harding (1921-1921)
2. Alfred Harding (d.1991)
others
Marriage: 15 Dec.1919 Dundela Presbyterian Church (by Jas Hunter), parish of Holywood, Co. Down, Alfred Chas. Harding, full age,
bach., coal miner, 6 Medway St., father-
Alfred Chas. Harding (died), to Abigail Duff, full age, widow, 51 Central St., father- Alex. Lyttle (joiner). Witnesses: Colin McKaig Rice, Emma Rice.
Roselawn Cemetery (U 423):
Abigail Harding, 76, 51 Central St., died 23 Feb.1966, buried 24 Feb.1966
Alfred Harding, 77, 51 Central St., died 28 Feb.1966, buried 1 Mar.1966
Wilhelmina Mercer, 58, 86b Newtownards Rd., died 25 Oct.1979, buried 29 Oct.1979
Alfred Harding, 67, 8 Tower Court, died 10 Dec.1991, buried 12 Dec.1991
https://ssl.belfastcity.gov.uk/burialsearch/BurialRecordDetails.aspx?RecordID=9163.3317