static void GiveNTFSPermissions(string folderPath,
                                string ntAccountName,
                                FileSystemRights accessRights)
{
    DirectorySecurity dirSecurity = Directory.GetAccessControl(folderPath);

    FileSystemAccessRule newAccessRule =
         new FileSystemAccessRule(
               ntAccountName,
               accessRights,
               AccessControlType.Allow);

    dirSecurity.AddAccessRule(newAccessRule);

    Directory.SetAccessControl(folderPath, dirSecurity);
}

GiveNTFSPermissions(@"\sri-devpcRootShare",
                    @"domainusername",
                    FileSystemRights.Write);

GiveNTFSPermissions(@"\sri-devpcRootShareInsideRootShare",
                    @"domainusername",
                    FileSystemRights.Write);

public void SetFileSystemRights(string target, string group, FileSystemRights permission)
    {
        if (!IsDirectory(target) && !IsFile(target))
            return;

        var oldSecurity = Directory.GetAccessControl(target);
        var newSecurity = new DirectorySecurity();

        newSecurity.SetSecurityDescriptorBinaryForm(oldSecurity.GetSecurityDescriptorBinaryForm());

        var accessRule = new FileSystemAccessRule(group,
                                                  permission,
                                                  InheritanceFlags.None,
                                                  PropagationFlags.NoPropagateInherit,
                                                  AccessControlType.Allow);
        bool result;
        newSecurity.ModifyAccessRule(AccessControlModification.Set, accessRule, out result);

        if (!result) Log.AddError("Something wrong happened");

        accessRule = new FileSystemAccessRule(group,
                                              permission,
                                              InheritanceFlags.ContainerInherit |
                                              InheritanceFlags.ObjectInherit,
                                              PropagationFlags.InheritOnly,
                                              AccessControlType.Allow);

        result = false;
        newSecurity.ModifyAccessRule(AccessControlModification.Add, accessRule, out result);
        if (!result) Log.AddError("Something wrong happened");

        Directory.SetAccessControl(target, newSecurity);

        if (result) Log.AddGood("Permissions set for '{0}' on folder '{1}'", group, target);

        if (!result) Log.AddError("Something wrong happened");
    }