The parentheses are a sidebar. Jacob mentions the Mighty One of Jacob and then adumbrates briefly about the Mighty One of Jacob (and thus the parenthesis in the translation).
The "Mighty One of Jacob" was the person with whom Jacob was wrestling in Genesis 32:24-30. (His hands were mighty.) When Jacob asked his name, the question was in fact superfluous, since the person had just already christened Jacob "Israel," which means "he strives/persists with God." In other words, Jacob deduced from his own new name and its meaning that he was actually striving with the Lord God, which is why he named the place "Penuel" according to Genesis 32:20. (Penuel means "the face of God," which Jacob had seen as the day had dawned.) Hosea the prophet confirms the same, that the person with whom Jacob had wrestled was none other than the Lord God (Hosea 12:4-5).
Now let's see how the parenthesis in Genesis 49:24 comes into play, which is a short sidebar concerning the HANDS of the Mighty One of Jacob.
First, from the hands of the Lord God (שָׁם = "from there") will come the "Stone of Israel." Here we have an immediate allusion to the pillow (stone) upon which Jacob slept, when he had his dream of the ladder ascending and descending upon him. That is, Jacob is the cornerstone, since the ladder from heaven was ascending and descending upon HIM, through whom the House of Israel would be built by the hands of the Mighty One. He is the "Stone of Israel," because he is the Promised Seed (descendant) of Abraham and Isaac upon whom the House of Israel would be built. Interestingly enough, Jacob called the place where he had his original dream "Beth-El," since the Most High One would make his dwelling among men on this cornerstone (who was Jacob). And so when he makes these sidebar comments concerning "the hands of the Mighty One of Jacob" in Genesis 49:24, Jacob is making allusion to his own seed, since his descendants are the 12 tribes of Israel. That is, Jacob is the line of the Promised Seed of Abraham, and he therefore comprises the cornerstone ("Stone of Israel") upon which the House of Israel will be built.
Second, there is also something else that comes from the hands of the Mighty One of Jacob: the "Shepherd." It is not until the Davidic Covenant that the individualistic aspect of the Promised Seed is finally "fleshed out" in the Hebrew Bible. That is, on the one hand the House of Israel is the collective promised seed of Abraham (plural sense), but on the other hand the "Shepherd" is the individual Promised Seed (singular sense), which (who) is "fleshed out" in the Davidic Covenant. This Promised Seed will be the Shepherd (king), who physically establishes the House of God on the cornerstone of Israel. King David actually uses the term "Mighty One of Jacob" TWICE in Psalm 132 when describing his own desire to establish the House of God (temple), which was to be fulfilled by his descendant, Solomon. Solomon was the partial fulfillment of the Davidic Covenant.
In other words, we have the dwelling of God among his chosen people (Israel), upon whom the Mighty One of Jacob built his presence. The Israelites are the collective seed of Jacob, and therefore they are the cornerstone ("Stone of Israel") on whom the Most High had established his presence among men. There is also the "Shepherd," who will come from the hands of the Mighty One of Jacob. He is the individual seed of Jacob (singular sense), who will be the shepherd of the House of Israel. This guide will have an eternal throne, and will therefore be the fulfillment of the Davidic Covenant. He will one day reunite the "lost sheep" of the House of Israel.