I found this surname information at https://listserv.heanet.ie/cgi-bin/wa?A2=ind9202&L=GAELIC-L&P=21373
From "The Surname of Ireland" by Edward MacLysaght (1973)
(O)DoolanIn Connacht this is O'Dubhlaing (in Irish):See Dowling.Doolan and Doolin are the anglicized forms of O'Dubhlainn (from dubhshlain' in Irish meaning challenge) in Munster, which in Leinster is made Dowling.Doolan is also an occasional synonym of Dolan in Connacht.
From "Irish Families" by Edward MacLysaght (4th Edition 1985)
(O)Dolan, DoolanThe name Dolan is fairly common today in Ulster - in the Catholic areas of Counties Cavan and Fermanagh - and in the Counties of Roscommon and Galway in Connacht.The latter is the place of origin of this sept which is a branch of the Ui Maine (Hy Many).In the census of 1659 the name appears principally in Counties Roscommon and Fermanagh (the portion dealint with Co. Galway is missing).The generally accepted form in Irish today is O'Dubhlain (mod. O'Dulain) as given by Woulfe and others.O'Doelan, later Dolan derives from the name O'Dobhailen, the name of a family on record since the 12th century in the baronies of Clonmacowen, Co. Galway, and Athlone, Co. Roscommon, in the heart of the Ui Maine country and quite distinct from the family of O'Doibhilin (Devlin).There has been a movement north-eastwards so that now the name Dolan is numerous in Counties Leitrim, Fermanagh and Cavan as well as in Counties Galway and Roscommon.The name Devlin in Counties Sligo, Leitrim and Cavan was widely changed to Dolan.Another anglicisation is Doolan and sometimes Dowling.A very well known Irish-American was Thomas Dolan (1834-1914), the capitalist; in Ireland the best known man of the name was Michael J. Dolan (d. 1953), an outstanding actor in the Abbey Theatre in Dublin.
My immigrant ancestor was Robert Dowling/Dooling/Duling born ca. 1680 in England or Ireland.