The original Gaelic is MacDhonnchaidh, "son of Donnchadh", a male given name sometimes englished as Duncan. The D sound is suppressed following the MAC, and this is shown in the written form by the letter H following the D. You will note the same letter combination at the end of the name, indicating the same effect. In written Gaelic, Irish or Scots, the letter H following another consonant indicates a change in pronunciation of the consonant. Often the sound is suppressed altogether. In the case of the C followed by H in the name Donnchadh the usually hard C sound is changed to the CH sound of words like loch. As near as I can get it Donnchadh would be pronounced something like Donnaha, and MacDhonnachaidh something like Maconnahee.