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Magnetar giant flares (MGFs), originating from non-catastrophic magnetars,share noteworthy similarities with some short gamma-ray bursts (GRBs). However,understanding their detailed origin and radiation mechanisms remainschallenging due to limited observations. The discovery of MGF GRB 231115A, thesecond extragalactic magnetar giant flare located in the Cigar galaxy at aluminosity distance of $\sim 3.5$ Mpc, offers yet another significantopportunity for gaining insights into the aforementioned topics. This Letterexplores its temporal properties and conducts a comprehensive analysis of boththe time-integrated and time-resolved spectra through empirical and physicalmodel fitting. Our results reveal certain properties of GRB 231115A that bearresemblances to GRB 200415A. We employ a Comptonized fireball bubble model, inwhich 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 thephotosphere radius. This leads to dynamic shifts in dominant emission featuresover time. Our model successfully fits the observed data, providing aconstrained physical picture, such as a trapped fireball with a radius of $\sim1.95 \times 10^{5}$ 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 confirmthe burst's MGF origin and its contribution to the MGF-GRB sample. We alsodiscuss prospects for further gravitational wave detection associated withMGFs, given their high event rate density ($\sim 8\times 10^5\ \rm Gpc^{-3}\yr^{-1}$) and ultra-high local magnetic field.

A Comptonized Fireball Bubble Fits the Second Extragalactic Magnetar Giant Flare GRB 231115A