The shoulder title is too small to be Irish Guards. The foot guards used larger letters and in combination with an icon of regimental insignia. This was a star of the order of St Patrick.
There does appear to be just two letters of the smaller size shown in the photo, which also rules out RE, RA, etc. who also used the larger size letters. Two letter combinations in the small size might be RS (Royal Scots), RH (Black Watch), in brass, and RB, SR, in blackened brass. The latter two are extremely unlikely as they relate to rifle regiments whose members usually (but not in every case) also wore black buttons.