The new-item
cmdlet is asking you to specify a type, because it doesn't know what $IRTPoolName
is.
Invoke-Command -ComputerName $targetName -ScriptBlock {
Param($IRTPoolName)
Import-Module WebAdministration
New-Item IIS:\AppPools\$IRTPoolName
} -ArgumentList $IRTPoolName -credential $cred -UseSSL