  Magnetar giant flares (MGFs), originating from noncatastrophic magnetars,
share noteworthy similarities with some short gamma-ray bursts (GRBs). However,
understanding their detailed origin and radiation mechanisms remains
challenging due to limited observations. The discovery of MGF GRB 231115A, the
second extragalactic MGF located in the Cigar galaxy at a luminosity distance
of $\sim 3.5$ Mpc, offers yet another significant opportunity for gaining
insights into the aforementioned topics. This Letter explores its temporal
properties and conducts a comprehensive analysis of both the time-integrated
and time-resolved spectra through empirical and physical model fitting. Our
results reveal certain properties of GRB 231115A that bear resemblances to GRB
200415A. We employ a Comptonized fireball bubble model, in which the Compton
cloud, formed by the magnetar wind with high density $e^{\pm}$, undergoes
Compton scattering and inverse Compton scattering, resulting in reshaped
thermal spectra from the expanding fireball at the photosphere radius. This
leads to dynamic shifts in dominant emission features over time. Our model
successfully fits the observed data, providing a constrained physical picture,
such as a trapped fireball with a radius of $\sim 1.95 \times 10^{5}$ cm and a
high local magnetic field of $2.5\times 10^{16}$ G. The derived peak energy and
isotropic energy of the event further confirm the burst's MGF origin and its
contribution to the MGF-GRB sample. We also discuss prospects for further
gravitational wave detection associated with MGFs, given their high-event-rate
density ($\sim 8\times 10^5\ \rm Gpc^{-3}\ yr^{-1}$) and ultrahigh local
magnetic field.
