http://www.houseofnames.com/xq/asp.fc/qx/rosson-family-crest.htm It would seem that your guess is correct, Domnhnaillín
Laus virtutis actio
Also, Rosson is not an Irish name, so a paraphrase rather than an original motto is the best we are going to get.
Molann suáilce gníomh
Praises * virtue * (an) action
This doesn't make a whole lot of sense
More speculation here
http://rawsonevents.com/chriswedding.html#creed Which ends up interpreting the Latin as
"The praise of manliness/character is action"
which would be
Moladh fireannachta gníomh
Praise * of manliness * action
But I'm out on a limb here....