A couple of secondary sources relating to Clan MacDougall:

• Barrow G, ‘The battle between John MacDougall of Lorn and Robert Bruce in 1308 or 1309’, (1999) 19 Notes and Queries of the Society of West Highland and Island Historical Research 3
• Duncan AAM and Brown AL 'Argyll and the Isles in the Earlier Middle Ages' (PDF) (1956) 90 Proceedings of the Society of Antiquaries of Scotland 192
• MacDonald AR, The kingdom of the isles: Scotland's western seaboard, c.1100–c.1336 (2nd edn, Birlinn 2011)
• MacDonald CM, History of Argyll (W & R Holmes 1950)
• MacDougall J, Highland postbag: the correspondence of four MacDougall chiefs, 1715-1865 (Shepheard-Walwyn 1984)
• Mackenzie A, History of the Macdonalds and Lords of the Isles (pdf) (A & W MacKenzie 1881)
• Macphail JRN (ed), Highland papers 2nd ser, vol 5, 1 (Scottish History Society 1914)
• Millett S, The heirs of Somerled: the historical origins of the MacDougalls and MacDonalds, 1100-1500 (Scottish Lore Press 1990)
• Raeburn A, 'How was territorial power and control in medieval Argyll expressed through its built heritage and did this change as the region moved from Scotland’s periphery to its core' (dissertation, University of the Highlands and Islands 2011)
• Sellar WDH, 'MacDougall pedigrees in MS 1467', (1986) 29 Notes and Queries of the Society of West Highland and Island Historical Research 3 (and you can access Manuscript 1467 here)
• Sellar WDH, 'MacDougall, John, lord of Argyll' in Oxford Dictionary of National Biography (2004)
• Sellar WDH, 'MacDougall, Alexander, lord of Argyll' in Oxford Dictionary of National Biography (2004)
• Sellar WDH, 'MacDougall, Ewen, lord of Argyll (d. in or after 1268), king in the Hebrides' in Oxford Dictionary of National Biography (2004)
• Sellar WDH, 'Hebridean sea-kings: the successors of Somerled, 1164–1316' in EJ Cowan and RAMcDonald (eds) Alba: Celtic Scotland in the middle ages (Tuckwell Press 2000), 187
• Starforth M, An Official Short History of Clan MacDougall (Scotpress 2015)

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See also the sources listed on Dubgall mac Somairle Wikipedia page. (You might be able find something relevant in The Kist, the journal of the Natural History and Antiquarian Society of Mid Argyll.)