1. Archers:
Depends. Very effective and deadly against very light armored and densely packed men or horses. Absolutly useless against platearmored footsoldiers which were able to march through a rain of arrows as it would be - rain.
Great disruptive effect against charging cavalry with unprotected horses - that's why steppe tribes started to protect their horses with heavy felt or linen since 500 BC.
Almost useless in killing heavy infantry in mail and with big shields.
The moral effect was significant and not just porportional to the damage they were able to inflict, especially when one lacked the ability to return in kind. Even the Spartans surrendered when light missile units, mostly peletasts surrounded them and pelted them from afar.
CBR points rightly out that the casuality rate must be higher in a TW game because among other factors it is unable to account for the confusion and the wounds suffered.
2. Javelineers:
Usually popular in areas with little other missile weapons, ideal for poor militas. Should be more effective against mail than an arrow but not particulary, especially when the distances are shortranged. Ireland was not an open country back tnan, but covered with forests afaik.
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