Santa Margherita di Antiochia is a 17th century former convent church in Trastevere, and hence is also known as Santa Margherita in Trastevere. The postal address is Via della Lungaretta 91/A, which is a side door. The main entrance is on the Piazza di Sant'Apollonia. Pictures of the church at Wikimedia Commons. 
The dedication is to St Margaret of Antioch, who by tradition was martyred in 305 although her legend is fictitious.
According to Panciroli writing in the 17th century, the church was founded by Pope Nicholas IV in 1288, and was then known as Santa Margherita della Scala. This was an unusual period for any church to be founded in Rome.
A convent of sisters of the Third Order of St Francis was established here around the start of the 16th century, which began as an informal unenclosed community of pious virgins of the sort familiar in northern Europe as beguines, but known in Italian as bizoche. The church was then also known as Sant'Elisabetta, after St Elizabeth of Hungary who had been a Franciscan tertiary.
As a result of scandals, the Church in Rome mostly ended up by requiring such unenclosed communities of women religious either to accept enclosure, or to disband, in the later 16th century. In 1564, the complex with the church was rebuilt with funds provided by Giulia Colonna, and rededicated to SS Margaret and Emygdius. In the process, the former tiny edifice with a façade on the Via della Lungaretta was rotated to stand parallel with that street, and doubled in size.
The Margaret in the dedication was changed by the sisters to St Margaret of Cortona, a famous mystic and Franciscan tertiary. It was changed back after they left. In 1572, the convent was occupied by a community of Dominican nuns which had moved from what is now Santo Spirito dei Napoletani.
Very oddly, on the same piazza was another, completely separate convent of Franciscan tertiary nuns at Sant'Apollonia who had also started out at bizoche. One wonders how the two Franciscan communities got on. The church there is gone, leading to the confusion caused by the church of Santa Margherita being on the Piazza di Sant'Apollonia.
The convent was sacked by the French during their occupation of the city at the end of the 18th century, and not re-founded.
In 1814, the complex was granted to the Confraternity of St Emygdius (Confraternita di Sant'Emidio). Later, for a short period, it was owned by the Pious Union of the Rosary of Pompeii, but was then given back to the former Confraternity of St Emygdius.
The convent was converted to private apartments in the later 19th century, but the church was restored in 1893.
On 2 July 2012, the Confraternity was suppressed by decree of the Ministro dell'Interno. This means that the church presently has no pastoral function. However, it remains administered by Mons. Giuseppe Tonello who has been rector-in-charge since 1997.
A restoration of the paintings is presently in progress (2014).
The church is parallel to the Via della Lungaretta on the north side. The apse is incorporated into a larger domestic building, and where the right hand roofline of the exterior wall meets this there is a small bellcote dating from the 19th century, with spaces for two bells.
The church has no aisles to the nave, but three side chapels. The exterior side wall that you see on the street conceals the two south chapels and a side entrance; there is a similar arrangement on the other side, except that there is only one chapel and the convent entrance. These two ranges are covered by single-pitched roofs.
If you compare the present interior with the plan shown in the Lanciani map (see links below), you will see that the presbyterium seems to have been altered and shortened at some stage, with the apse being moved forward. This happened before 1839, when Nibby described the present arrangement.
FaçadeEditThe façade was designed and executed by Carlo Fontana as part of the rebuilding between 1678 and 1680; he was commissioned by Girolamo Cardinal Gastaldi. The project involved rotating the church by ninety degrees, so as to have the façade present a monumental aspect on the piazza.
There are two storeys, rendered in pale orange with white architectural detailing. At first glance, it seems that the first storey fronts a nave with side aisles. As mentioned, this is misleading; there are no aisles, which is why there is only one entrance.
The central vertical section of the façade is brought forward slightly, and in the first storey has two pairs of Composite pilasters with the outer pair doubletted round the corner. These stand on a very high plinth. The tall central doorway has a blank curved trapezoidal tablet above it, which is sheltered by a floating archivolt with its ends curved under in curlicues -a playful detail typical of the architect. The two, narrower side zones have a pilaster each on the outer corner, again doubletted round the corner, and each has a large empty round-headed niche.
The six pilasters support an entablature with a dedicatory inscription on the frieze. It reads: In honorem S[anctae] Margheritae V[irginis] et M[artyris] et S[ancti] Emigdii Ep[iscopi] et M[artyris].
The second storey has a large rectangular window with a strongly projecting cornice, flanked by two pairs of pilasters lacking capitals. On top is a triangular pediment with an empty tympanum. On either side are what look like gigantic volutes, but a second glance will reveal that they are actually a pair of gigantic tondi with attached upsweeping curves. The architect was having fun again.
The church has a single nave, barrel-vaulted with rather restrained Baroque decoration mostly in white. There are three bays of unequal depth, the central one being wider and containing a pair of side chapels. The shallower bays have balconied galleries for the nuns, high up with balustrades. The bays are separated by double Corinthian pilasters, and there are four such pairs at the corners of the nave with one of each folded into the corner.
The barrel vault has transverse ribs above the pilasters in the side walls, and lunettes for three windows on each side (the central one again being larger). There is no decoration.
There were originally two side chapels, but in 1892 a third was added on the right-hand-side.
The 19th century presbyterium apse is a complete contrast, as it is panelled sumptuously in what looks like different kinds of rare polychrome marble -which may be fake. The presbyterium arch is supported by one and a half Corinthian pilasters on each side, in a brown and white veined marble which looks un-natural. The fake marble that church restorers used is called scagliola, and there is a lot of it in Roman churches masquerading as the real thing.
The high altar is against the wall of the apse, and has no canopy. The altarpiece by Giacinto Brandi depicts St Margaret in Prison, Envisioning the Holy Cross. The (completely unhistorical) story behind this is that she had been incarcerated in a cell in the ancient city wall near the Lateran, and this location gave rise to a little pilgrimage chapel -Santa Margherita in Prigione. The painting is framed in what looks like verde antico marble.
To the sides are a pair of elliptical paintings by Giuseppe Ghezzi depicting The Martyrdom of St Margaret to the left, and The Martyrdom of St Apollonia to the right.
The conch of the apse, framed by the archivolt of the triumphal arch, contains a fresco of the Assumption by a talented Franciscan friar, Francesco Umili da Foligno.
The two original nave side chapels are shallow arched recesses, containing polychrome marble altar aedicules of slightly different designs.
In the chapel on the left is a painting by Il Baciccio, depicting The Immaculate Conception with SS Francis and Clare. The aedicule has a pair of doubletted Corinthian pilasters in what looks like a brown brecciated marble, and these do not support a pediment but an arc cornice on two posts. These bear good stucco decoration involving angels, putti and garlands.
The nave chapel on the right has an altarpiece depicting St Emygdius. The aedicule has no stucco ornament, and the arc cornice is slightly raised.
The 19th century room chapel has a painting of St Ursula by Paolo Severi.
Details of access and liturgical activities are not easily found.
The feast of St Margaret of Antioch is celebrated with solemnity on 20 July.