I decided to make a list of some words in the Irish translation of Harry Potter that I don't know and I found one that was not in my "Pocket Oxford IRISH Dictionary". The word is sómhlófá. What does this word mean? Is this an obscure word or is this not a very good dictionary?

I had to look it up, but -- unless it's a misprint for "samhlófá" ("you would imagine"), which I suppose it almost certainly is -- the answer is: "you would consume" or "you would devour."

Niall Ó Dónaill's dictionary has:

somhaltas = TOMHALTAS

tomhail, v.t. & i. Eat, consume.

tomhailt, f. (gs. ~e). 1. vn. of TOMHAIL. 2. Consumption (of food, drink).

tomhaltach, a1. Consuming; eager for food.

tomhaltas, m. (gs. -ais). 1. Consumption of food. 2. Eatables, food; meal.

tomhaltóir, m. (gs. -óra, pl. ~í). Eater, consumer; big eater.

Dinneen has:

Tómhlaim, vl. -mhailt, v. tr., I eat, devour, consume; tomhailim (-Donlevy's Irish Catechism), toimhilim (-Ancient Laws of Ireland)
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In Rudolf Thurneysen's "A Grammar of Old Irish" one finds:

tomil "eat!" (sg.)
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But, as I said, the word in question is probably supposed to be "samhlófá"; the context will tell you.

"Samhlófá" is not obscure. "Sómhlófá" is either obscure or nonexistent, and thus is highly suspect.

No, the Oxford Pocket Irish Dictionary is not great, but it's all right to use until you're a little farther along and you decide it's time to get something a lot more complete and a lot more accurate.