Hi Anne,
There only appears to be one tour company that leaves from Malahide, The majority leave from Dublin.
They are called Grand Tours and you can
find them here.
They are not registered on Ireland.com and I have not heard any reviews on them, as they appear to be a relatively new company. You may want to have a google and check their reputation before booking.
Hopefully, it works out for you
Regards,
Gordee