This is a very interesting question. The source seems to be a pre-Norman Conquest Irish poem, presumably written in the Gaelic Celtic language. Seemingly, the original poem derives from the 1000s (11th century). It seems to be a poem written by Patrick, Bishop of Dublin (c. 1084). Patrick's poem was later rendered into a Latin paraphrase, a "thirteenth-century Latin poem on the Wonders of Ireland." Presumably, Patrick's poem collected Irish folk lore and legend to preserve it or popularize it.

The extant thirteenth century (1200s) Latin paraphrase speaks of strange Irish phenomenon like "ships floating in the air." Included in these phenomenon is the tale of Irish wolves, or people given to lycanthropy, or werewolfism. The Latin paraphrase of the earlier Patrick poem describes a very civilized species of Irish werewolves. They separate from their human body, which they ask their friends to carefully guard, because if their bodies are moved in the slightest, the Irish wolf can never return to human form. Then the re-embodied werewolves go off to eat sheep, not humans.

So when Rosalind exerts herself to quell the love bickering between Silvius, Phebe, Orlando and herself, she says they are making a sound like Irish wolves, which, if they are attacking a herd of sheep or even looking for a herd of sheep to attack, must be a considerable and fierce noise.

PHEBE
    And so am I for Ganymede.

ORLANDO
    And so am I for Rosalind.

ROSALIND
    And so am I for no woman.

PHEBE
    If this be so, why blame you me to love you?

SILVIUS
    If this be so, why blame you me to love you?

ORLANDO
    If this be so, why blame you me to love you?

ROSALIND
    Who do you speak to, 'Why blame you me to love you?'

ORLANDO
    To her that is not here, nor doth not hear.

ROSALIND
    Pray you, no more of this; 'tis like the howling
    of Irish wolves against the moon.

[eNotes does not usually endorse the use of blogs, but this one has the only reference available for "Irish wolves" and manifests expert knowledge:  Karl Steel. "Tis Like the Howling of Irish Wolves Against the Moone." 12 March 2007. In the Middle. J J Cohen Blog.]