Well they won't do you any good, no wonder you lost.
I'm hoping they were more than 50% of your army. If not, there are serious issues in your military system.
Low discipline, low morale, etc.
I'm assuming you're Rome, Epirus's commander was probably Pyrrhus (9-star), and Epirus's army is always a mix of heavy infantry, bowmen, and both heavy and archer cavalry. It's no wonder you lost, bringing velites to a battle like that.
You might have also made the mistake of landing your men right in the province, giving them a negative from both terrain, and amphibious landing. Try getting military access and landing them in a country next to Epirus.
Hover over the troops' icons to see their type, their bonuses due to omens, laws, events, etc., and their effectiveness against all other types of troops. Velites are 100% effective against velites, and less than 100% against everything else (very bad, they're the worst troop type, fit only for arrow fodder and only an option when you have no money); every other troop type (heavy infantry, cavalry, elephants, etc.) has a bonus (more than 100% effectiveness) against 1 or more of the other types.
A little story related to this:
My first ever game was as Sparta. I took a National Idea that increases heavy infantry discipline by 10%, and enacted a Law that increases heavy infantry discipline by 20%. My cash and force limits only enabled me to have 2,000 men, and Aetolian League suddenly attacked me...with 13,000! I thought I was done for, but I didn't ragequit. I put my 7-star king in command and waited for the inevitable.
The battle was amazing. Every day I'd lose 5 men, the enemy would lose 500. And that's when I learned that a general's stars, the terrain, troop type, and morale and discipline MATTER. This isn't CK2.