Cleanly setting ACLs
To streamline the permission handling you may want to wrap ACE creation in a function and set the ACL just once at after all ACEs were added:

function New-Ace {
  [CmdletBinding()]
  Param(
    [Parameter(Mandatory=$true, Position=0)]
    [Security.Principal.NTAccount]$Account,
    [Parameter(Mandatory=$false, Position=1)]
    [Security.AccessControl.FileSystemRights]$Permissions = 'ReadAndExecute',
    [Parameter(Mandatory=$false, Position=2)]
    [Security.AccessControl.InheritanceFlags]$InheritanceFlags = 'ContainerInherit,ObjectInherit',
    [Parameter(Mandatory=$false, Position=3)]
    [Security.AccessControl.PropagationFlags]$PropagationFlags = 'None',
    [Parameter(Mandatory=$false, Position=4)]
    [Security.AccessControl.AccessControlType]$Type = 'Allow'
  )

  New-Object Security.AccessControl.FileSystemAccessRule(
    $Account, $Permissions, $InheritanceFlags, $PropagationFlags, $Type
  )
}

$domain = 'DOMAIN'

$acl = Get-Acl $path

'Administrators', "$domain\Domain Admins" | ForEach-Object {
  $acl.AddAccessRule((New-Ace $_ 'FullControl'))
}
$acl.AddAccessRule((New-Ace "$domain\FS-$NAME-RW" 'Modify'))
$acl.AddAccessRule((New-Ace "$domain\FS-$NAME-R" 'ReadAndExecute'))

Set-Acl $path $acl


Other improvements might include determining the administrators and domain admins group names independent of the system's locale by their well-known SIDs, e.g.:

$administrators = ([wmi]"Win32_Sid.Sid='S-1-5-32-544'").AccountName

$domain_admins  = Get-WmiObject -Class Win32_Account |
                  Where-Object { $_.SID -like '*-512' } |
                  Select-Object -Expand Caption


Another thing you could do is map names to permissions like this:

$permissions = @{
  'Administrators'        = 'FullControl'
  "$domain\Domain Admins" = 'FullControl'
  "$domain\FS-$NAME-RW"   = 'Modify'
  "$domain\FS-$NAME-R"    = 'ReadAndExecute'
}


and use the hashtable as input for the ACE creation:

$permissions.Keys | ForEach-Object {
  $acl.AddAccessRule((New-Ace $_ $permissions[$_]))
}


Of course this implies that all ACEs have the same type and flags, otherwise you'd have to use a more elaborate data structure.