Can't find a record of his middle name, but it was probably Shimmin.

He appears to be the son of Edmund Garrett and Mary Kelly, baptized 26 Feb 1804 at German.

Siblings
Thomas 1796
Charles 1798
Catherine 1801

All on the IGI

1851, Patrick, district 3, schedule 49, Rheaby
Edmund Garrett, head, married, 44, agricultural labourer, German
Catherine, wife, married, 40. German
Edmund, son unmarried, 14, lead ore washer, German
Thomas, son, 11, scholar, German
Elizabeth, daughter, 9, scholar, German
William, son, 7, scholar, German
Jane, daughter, 5, German
John, son, 1, Patrick