Usage of this name actually predates its usage in Edna Lyall's novel by over a decade. There was a lot of coverage in papers in 1878 regarding the marriage of Lady Dorothy Blanche Boyle, daughter of the 9th Earl of Cork, to Walter Hume Long, 1st Viscount Long. Lady Boyle was known by the - seemingly invented - nickname 'Doreen'. Thanks to the wedding coverage, 1878 marked the births of the first true Doreens, with 3 girls receiving it as a first name and 3 as a middle name that year. [noted -ed]
I saw one source list this name as coming from Irish Doireann, of which it said that Doireann its own turn came from Dorothea (Dorothy). Now, perhaps the origin listed in the database is the only correct one, but I think the similarities with the aforementioned names have caused people to use e.g. Doreen as a nickname or short form for Dorothy. I do know that this is the case in my country (The Netherlands), where names of Gaelic origin have never quite been so popular (historically speaking) and therefore people were more likely to use Doreen and especially Dorine as a short form for Dorothea. Dutch books and websites therefore also list these as coming from Dorothea, so perhaps there is some truth to the possibility of Doreen and Dorine coming from Dorothea. Just thought this might be of use to you. :)
Adding the suffix "een" is an Irish thing, means "little" or something diminuitive. For example, "Pegeen" or "Michaeleen" etc.
― Anonymous User 9/3/2007
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It is thought that this name is from a Gaelic word meaning daughter. Doreens are said to be strong in both body and mind. It is a common name in Germany and France but is spelt Dorene.