Hello Helen, I looked in McLysaghts book Surnames of Ireland, and it says Scollan is the Leinster form of the name called Scullin in Ulster, when I then look at Scullin it gives variants as Scullane; Scullion; Skollin and in Gaelic O'Scullain, there are fadas (Irish language for a back slash over the letter and it occurs over the "O" and the "a" in the name so oh Scull aain in sound is the best I can do. A Derry erenagh (no idea what that is) sept (Irish clan) from Ballyscullion (Derry) whose name is sometimes changed to Scully, it is not a variant of Scallan. It is very rare outside Ulster (Leitrim is near the province of Ulster and Derry also. It refers to his other book More Irish Families but I do not have a copy.
There is a site here that is subscription but you can search the surnames for free for a few times. https://www.johngrenham.com/surnames/ there is also a Facebook page The Irish Surname Registry think they may haves changed it to Irish Surnames Registry and DNA or somethink similiar so you may find someone there who knows more.
You will also find the name on this site of a Scollan family and other Leitirm families.