The reliance of deep learning algorithms on large scale datasets is a significant challenge for sign language recognition (SLR). The shortage of data resources for training SLR models inevitably leads to poor generalisation, especially for low-resource languages. We propose novel data augmentation and preprocessing techniques based on synthetic data generation to overcome these generalisation difficulties. Using these methods, our models achieved a top-1 accuracy of 86.7% and a top-2 accuracy of 95.5% when evaluated against an unseen corpus of Irish Sign Language (ISL) fingerspelling video recordings. We believe that this constitutes a state-of-the-art performance baseline for an Irish Sign Language recognition model when tested on an unseen dataset.