Hi Phillip,
You would cite Hearth Money Roll for County Donegal, 1665. TCD MS 6232. Trinity College, Dublin, Manuscripts and Archives Research Library. Then list the name of the taxpayer (i.e. John Alexander), the barony, parish, and townland where he is located, the number of hearths he was taxed for, and the page number where the entry was found (with a link to the record). As long as you provide as much information as possible so others can verify the source, you will be good.
Jen, Ireland Project co-leader