Sometimes it works out with just adding the pack. I would suggest making a starter though. Out of the couple of brews i've made where i only added one pack of yeast, a few of them never dropped below 1.020. This could be related to other factors (temps) as opposed to yeast cell count, but i haven't had any batches fall short of the recipe's FG since i began utilizing a starter wort.
Congrats on the Irish Red! I've got a keg of Irish Red that has been carbing for 2 weeks. Going to tap into that bad boy tonight. CHEERS!