It is from the Gaelic Gleann Siara (Wikipedia so better check the spelling!!!) and wiki claims it means glen of the eternal river (wiki again so - check!) There are quite a few articles and pictures and engineering info if you google glen shira dam. I vaguely recall somewhere else reading about the grave of a dog being somewhere on the the roadside up to the dam- -- Perhaps also Neil Munro wrote about the glen? -- prior to the dam which was completed in 1956. He was a novelist and poet who was born and raised in Inveraray, became the editor (I think) of the Glasgow Herald in the days when it was a well respected newspaper (unlike now sadly)
If you are into dams then check Cruachan hydro scheme - they were building that when I was a student working in Inveraray, it is the most amazing scheme with a dam and lochan high up in a corrie and the hydro power station in a huge carved out hollow within the mountain. when electricity not required the turbines reverse and pump water back up the mountain to replenish the lochan! I don't remember any songs about it tho there may well have been some - but there were plenty of stories!