DED are District Electoral Divisions or small election districts. The three areas/islands of Lettermore, Gorumna and Lettermullen make up the DED named Gorumna.
The DED for the Aran Islands is Inishmore.
Our O'Malley and Conneely ancestors came from Gorumna, but there are quite a few Conneely's from Inishmore. The Aran Isles are larger and have about 1/3 more population than Lettermore, Gorumna and Lettermullen.
The Aran Isles also had a greater religious diversity. In 1901 the DED of Gorumna was 99% Catholic while Inishmore was a mere 97% Catholic.
Folks didn't move around much, just about the same percentages from the 1901 Cencus to the 1911 Census that were born in Galway and stayed in Galway.
The attached files are a couple of spreadsheets I put together from the 1901 & 1911 Irish census for Gorumna & Inishmore.
DED Gorumna 1901 & 1911 DED Inishmore 1901 & 1911
DED GORUMNA
DED INISHMORE
