Tirzah Chapel is dated to 1905. It was a daughter chapel of Trinity, Penclawdd, serving the local coalmining community of Llanmorlais, and was at first considered principally as a Sunday School. The original site lease from Sir Robert Armine Morris to Seth Protheroe Jones and others, trustees of Tirzah, is dated 1908, and gives permission to build to the value of £250. In 1931 the chapel acquired its independence from Trinity, and at the same period the trustees acquired the freehold of the chapel site.

Tirzah is a small chapel with a rendered exterior and a slate roof with a small finial at each end. The front is pedimented and has a plinth and rusticated quoins worked in the render. There is a central doorway with one window to each side, all with keyed arches and rusticated jambs also worked in the render. The windows have timber Y tracery, and timber glazing bars. There are double doors with two-panel leaves and similar glazing bars in head. The side elevations have segmental-headed window openings with timber mullion and transom windows. The front carries the chapel name Tirzah in the pediment and the wording 'Baptist Church Built 1905' beneath, in raised lettering.

There is a well preserved interior entered by a framed-pine entrance porch with moulded cornice, and double panelled doors to the interior. Above is a transom window with obscured glazing borrowed light above, and similar panelling and glazing at the sides of the porch.

The interior features a full-width, combined pulpit and organ loft with a single staircase to the left. At the rear is the organ in a shallow round-headed recess decorated with pilasters and an arch, below which is the wording 'Enter into His courts with praise' painted red. The seating consists of school benches not fixed to the floor. The ceiling is boarded at collar beam level in four main panels decoratively stencilled at corners, the panels defined by ribs, and with a central lozenge containing a ventilator.
(Source: Cadw Listing Database) RCAHMW 17th April 2008