Just looking at the notes, this could be an augmented 6th chord (my suspicions were aroused when I saw the C♯).
Augmented 6th chords are built from a triad with the augmented 6th interval, so E♭-G-B♭-C♯ (note: this is enharmonic to an E♭7 chord, but it's written with the A6 instead of the m7 interval). This describes the German Augmented 6th. There's another type called the Italian Augmented 6th, which is just like the German but lacks the fifth (E♭-G-C♯) That sounds pretty close to what you just described.
Possibly the French augmented 6th, which has E♭-G-A-C♯, changing the 5th to an augmented 4th. In your example, that A appears in the next bar, so maybe that's it.
The case against this being a +6 chord would be that this doesn't resolve like one. Usually, these chords would be built on the ♭VI and resolve to the V chord. However, this is built on the ♭II and resolves to the I. Similar, and perhaps this is a temporary tonicization of the IV chord, G major, but possibly not. It could be simply a new use for the +6 chord, but that brings into issue spelling.
So far, I've only been considering that the notes are all spelled correctly in the sheet music. If they're not, then my best guess is either a tritone substitution for the dominant or a Neapolitan 6th chord leading back to D. That, or A7♯11. A lot of the analysis will depend on genre, evidently. (Actually, that A7♯11 would be spelled correctly, so if this were jazz, this would almost certainly be correct.)
Side note: The different types of Neapolitan 6th chords can be chalked up to voice-leading. In C major, Gr+6 is A♭-C-E♭-F♯, which resolves to G:
- A♭ to G
- C to B
- E♭ to D
- F♯ to G
The It+6 is the same, but getting rid of the E♭ to D (no parallel 5ths!). Usually, the E♭ turns into another C which resolves up to D.
The Fr+6 replaces the E♭ with a D, resolving it to D, obviously. This also solves that parallel 5ths violation that the Gr+6 has (though I believe it was used anyway, composers treated it as an exception to that rule).
To sum up, the Gr+6 resolves its 5th down a half step, the It+6 resolves it up a whole step, and the French leaves it alone to get to the chordal fifth on the dominant chord.