Hi has anyone used the kennels on either of these and would they recommend them?

I've not used the kennels, but have taken the car on with dog in the car.  You can leave dogs in cars and check on them with staff accompanying you.

Not used them personally but friends do use the Holyhead - Dublin ones.  Some of the ferries have good safe plastic and steel type kennels.  But at least one ship has only metal cages on the ground in an area of a car deck with only a curtain between the kennels and the cars.  We were on a ferry last year and dog got out of one of these and was lose on the car deck for some time.