We start with product manifold $ X := S_g \times (0,1)$, where $S_g$ is a closed, orientable surface of genus $g \geq 1$. Since this has $\mathbb R^3$ as its universal cover, $X$ is irreducible. Now $X$ can also be seen as the interior of $\bar{X} = S_g \times [0,1]$. We want to manipulate $X$ a bit in order to create a bordism between $S_g$ and $S_{g+1}$, whose interior is ,hopefully, also an irreducible manifold.

Consider a boundary component $R := S_g \times \{0\}$ and fix a 2-disc $D \subset R$. This disc has an open subset $U \subset \bar{X}$ that is diffeomorphic to $D \times [0,\epsilon)$, such that $U \cap R = D$ (This follows since $R$ has a collar). Choose two distinct points $p,q \in D$ and connect them via a simple (injective) smooth arc $\gamma \subset U$ with the property that $\gamma \cap R = \partial \gamma = \{p,q\}$. Moreover, to ensure that $\gamma$ is as "simple" as possible, we additionally require that there exists a simple path $\gamma' \subset D$ between $p$ and $q$ such that the simple closed curve $\gamma \cup \gamma'$ bounds a disc $D' \subset U$, intersectig $D$ only in its boundary arc $\gamma'$. Drawing a picture isn't hard and really helps to make this paragraph more clear.

Now consider a tubular neighborhood $N \subset U$ of $\gamma$, such that $N \cong D_2 \times [0,1]$, where $D_2$ denotes the standart *open* unit disc of $\mathbb R^2$ and define $\bar{Y} := \bar{X} \setminus N$. Effectively, we have removed an open half-tube from $\bar{X}$ and simultaneously added a handle to the boundary component $R$, thereby increasing its genus by $1$. Hence, $\bar{Y}$ describes a bordism between $S_g$ and $S_{g+1}$ and, up to diffeomorphism, it doesnt depend on the exact choices of $D,p,q,U$ and $\gamma $. If $Y$ denotes its interior, is it clear that $Y$ is still irreducible ?

Remarks:

If $\tilde{Y}$ denotes the universal cover of $Y$, then $Y$ is irreducible if and only if $H_2(\tilde{Y}) = 0$, which follows from Hurewicz and the sphere theorem.

Knowing that $\mathbb R^3$ covers $X$, what can be said about $\tilde{Y}$? Since $Y$ can be regarded as an open subset of $X$ and $\mathbb R^3$ covers $X$ via some map $p$, we know that each component of $p^{-1}(Y)$ is a covering space of $Y$. What can be said about those components ?