Я понял, тип решения - способ решить эту проблему.
func getsomedata(issue_id string) {
issue, _, _ := jiraClient.Issue.Get(issue_id, nil)
// This returns as a slice of interfaces []interfaces{}
data := issue.Fields.Unknowns["customfield_12345"]
// Will use the length of the slice in the for loop below
s := reflect.ValueOf(data)
// For each index index in the slice, do...
for i := 0; i < s.Len(); i++ {
// Use type assertion to get the value I need for all indexes "i"
d := data.([]interface{})[i].(map[string]interface{})
fmt.Printf("%v\n", d["key"].(string))
}
}