I don,t necessarily disagree but even 'accidental' contacts with arms are penalised so why not head contacts particularly if the opposition is taken out?

Again a fair point but similar to the tackled player falling to ground in a tackle round the legs gets caught high by another tackler who originally was going lower - the second tackler still gets penalised.

Also suggest that the decision becomes referee subjective and we can do with less not more of these situations.

Must say this is not a specific attack on Monaghan, just used that incident, accidental or otherwise, as an example.

It's a contact sport. Unfortunately that means this sort of incident is always going to happen.

but why then is an arm to the head a penalty, when clashes of heads are not, as surely the same could be said of those?

If the incident is the one I think it is .. Would anyone deliberately go in for a headbutt on Sinny's brow ?

Because in the context of a tackle the arms are relatively free to move in multiple directions/angles under control from the player when effecting a tackle, where those arms should be in relation to a players body can be easily controlled.
Then look at the head, it is fixed in place with a maximum of 6-8 inches sideways movement, it can be extremely hard to move the head out of the way when the momentum of your body is the main deciding factor in its trajectory. No-one tries to make a tackle with their head (it fecking hurts banging your head) nor have I seen anyone run in leading with their heads to make deliberate contact to another players head (okay not for over 25 years and then the player was a bit mental to say the least)
Those saying MM deliberately used his head against Sinfield are barking and I've yet to see anything but clear heabutts (when standing & not running in) as the only time contact to the head with another head should be a penalty.